public class Startup
{
    // ...

    public void ConfigureServices(IServiceCollection services)
    {
        // ...
        services.AddControllers().AddNewtonsoftJson();

        services.AddMediatR(typeof(Startup).Assembly); // And any other assembly that may be needed.
        
        services.AddNavigator(options =>
        {
            options.SetTelegramToken(Configuration["BOT_TOKEN"]); // Your telegram bot token.
            options.SetWebHookBaseUrl(Configuration["BASE_WEBHOOK_URL"]); // The base url where you are going to receive the updates from teelgram.
            options.RegisterActionsFromAssemblies(typeof(Startup).Assembly); // All your actions.
        });

        /// ...
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            /// ...

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
                endpoints.MapNavigator(); // In order to receive updates from telegram.
            });

            /// ...
        }
}

public class EchoAction : MessageAction
{
    public string MessageToEcho { get; set; } = string.Empty;
    
    public override IAction Init(INavigatorContext ctx)
    {
        if (string.IsNullOrWhiteSpace(ctx.Update.Message.Text))
        {
            MessageToEcho = ctx.Update.Message.Text;
        }
        return this;
    }

    public override bool CanHandle(INavigatorContext ctx)
    {
        return !string.IsNullOrWhiteSpace(MessageToEcho);
    }
}

/// ...

public class EchoActionHandler : ActionHandler<EchoAction>
{
    public EchoActionHandler(INavigatorContext ctx) : base(ctx)
    {
    }

    public override async Task<Unit> Handle(EchoAction request, CancellationToken cancellationToken)
    {
        await Ctx.Client.SendTextMessageAsync(Ctx.GetTelegramChat(), request.MessageToEcho,
            cancellationToken: cancellationToken);

        return Unit.Value;
    }
}