I just created an empty hello world project and included liboai, nlohmann-json and cURL to it.
1>Blade.obj : error LNK2019: unresolved external symbol "public: virtual __cdecl liboai::netimpl::Session::~Session(void)" (??1Session@netimpl@liboai@@UEAA@XZ) referenced in function "class liboai::Response __cdecl liboai::netimpl::Delete<class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&>(class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$Delete@$$QEAVUrl@components@netimpl@liboai@@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YA?AVResponse@1@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
1>Blade.obj : error LNK2019: unresolved external symbol "public: class liboai::Response __cdecl liboai::netimpl::Session::Get(void)" (?Get@Session@netimpl@liboai@@QEAA?AVResponse@3@XZ) referenced in function "class liboai::Response __cdecl liboai::netimpl::Get<class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&>(class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$Get@$$QEAVUrl@components@netimpl@liboai@@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YA?AVResponse@1@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
1>Blade.obj : error LNK2019: unresolved external symbol "public: class liboai::Response __cdecl liboai::netimpl::Session::Post(void)" (?Post@Session@netimpl@liboai@@QEAA?AVResponse@3@XZ) referenced in function "class liboai::Response __cdecl liboai::netimpl::Post<class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&>(class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$Post@$$QEAVUrl@components@netimpl@liboai@@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YA?AVResponse@1@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
1>Blade.obj : error LNK2019: unresolved external symbol "public: class liboai::Response __cdecl liboai::netimpl::Session::Delete(void)" (?Delete@Session@netimpl@liboai@@QEAA?AVResponse@3@XZ) referenced in function "class liboai::Response __cdecl liboai::netimpl::Delete<class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&>(class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$Delete@$$QEAVUrl@components@netimpl@liboai@@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YA?AVResponse@1@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
1>Blade.obj : error LNK2019: unresolved external symbol "private: void __cdecl liboai::netimpl::Session::SetOption(class liboai::netimpl::components::Url const &)" (?SetOption@Session@netimpl@liboai@@AEAAXAEBVUrl@components@23@@Z) referenced in function "void __cdecl liboai::netimpl::set_options<class liboai::netimpl::components::Url,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > >(class liboai::netimpl::Session &,class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$set_options@VUrl@components@netimpl@liboai@@V?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YAXAEAVSession@01@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
1>Blade.obj : error LNK2019: unresolved external symbol "private: void __cdecl liboai::netimpl::Session::SetOption(class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > const &)" (?SetOption@Session@netimpl@liboai@@AEAAXAEBV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z) referenced in function "void __cdecl liboai::netimpl::set_options<class liboai::netimpl::components::Url,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > >(class liboai::netimpl::Session &,class liboai::netimpl::components::Url &&,class std::map<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,struct liboai::netimpl::components::CaseInsensitiveCompare,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > > &&)" (??$set_options@VUrl@components@netimpl@liboai@@V?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@netimpl@liboai@@YAXAEAVSession@01@$$QEAVUrl@components@01@$$QEAV?$map@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@UCaseInsensitiveCompare@components@netimpl@liboai@@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V12@@std@@@2@@std@@@Z)
I must add that this is actually my first C++ project, so the error might be entirely on my side.