You can follow the tutorial here :
https://lalitacode.com/how-to-implement-tenant-selection-in-identity-server-4-application/
if you want to follow the video your can visit this link below :
To run this application follow the points belows :
Add ConnectionString in your appsettings.json file with "DefaultConnection" connection to your database.
Make the required changes to the database seed in IdentityDbcontext, MasterDbContext and ConfigurationDbContext. You need to give your client configuration as per your requirement.
Run the migrations for IdentityDbContext, PersistedGrantDbContext, MasterDbContext and ConfigurationDbContext and update database.
Make changes to the client config in app.module.ts as per your requirement.
Finally run the application.