Token Introspection for reference token in .Net 7 Core Web API

[ILoveCoding1100]

Environment
I started the app in Visual Studio on my local PC. I'm using .Net 7 with IdentityModel.AspNetCore.OAuth2Introspection" Version="6.1.0"

Question 1
If I use the following code:
builder.Services.AddAuthentication("token").AddOAuth2Introspection("token", options => { options.Authority = authority; options.ClientId = clientId; options.ClientSecret = clientSecret;
If I call end endpoint with postman (with an reference token). I get the following error:

System.InvalidOperationException: Discovery endpoint https://xxx/as/introspect.oauth2 is unavailable: Error connecting to https://xxx/as/introspect.oauth2/.well-known/openid-configuration: Not Found
at IdentityModel.AspNetCore.OAuth2Introspection.PostConfigureOAuth2IntrospectionOptions.GetIntrospectionEndpointFromDiscoveryDocument(OAuth2IntrospectionOptions options)
at IdentityModel.AspNetCore.OAuth2Introspection.PostConfigureOAuth2IntrospectionOptions.InitializeIntrospectionClient(OAuth2IntrospectionOptions options)
at IdentityModel.AspNetCore.OAuth2Introspection.OAuth2IntrospectionHandler.LoadClaimsForToken(String token, HttpContext context, AuthenticationScheme scheme, OAuth2IntrospectionEvents events, OAuth2IntrospectionOptions options)
at IdentityModel.AspNetCore.OAuth2Introspection.OAuth2IntrospectionHandler.<>c__DisplayClass8_0.<b__2>d.MoveNext()
--- End of stack trace from previous location ---
at IdentityModel.AspNetCore.OAuth2Introspection.OAuth2IntrospectionHandler.HandleAuthenticateAsync()
at Microsoft.AspNetCore.Authentication.AuthenticationHandler`1.AuthenticateAsync()
at Microsoft.AspNetCore.Authentication.AuthenticationService.AuthenticateAsync(HttpContext context, String scheme)
at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

I know it should work. Because other applications are using this token introspection endpoint as well. Am I missing something?

I added this snippets to my code:

builder.Services.AddAuthentication("token").AddOAuth2Introspection("token", options => { options.Authority = authority; options.ClientId = clientId; options.ClientSecret = clientSecret;

if (isAuthenticationEnabled) { app.UseAuthentication(); }

[Authorize] above my Controller
Is that all to get the authentication up and running in principle?

Question 2
Is there a difference between:
services.AddAuthentication(OAuth2IntrospectionDefaults.AuthenticationScheme) .AddOAuth2Introspection(options =>

and

builder.Services.AddAuthentication("token").AddOAuth2Introspection("token", options =>

Question 3
In the end I need a user context for a valid reference token.
The following article describes how to get user information. What is not clear to me is which approach to use now? The one from Question 1 or the approach from the following article.
https://identitymodel.readthedocs.io/en/latest/client/introspection.html

[brockallen]

Do you have a call stack anywhere that you can share?

[ILoveCoding1100]

@brockallen I updated the post.

[brockallen]

System.InvalidOperationException: Discovery endpoint https://xxx/as/introspect.oauth2 is unavailable: Error connecting to https://xxx/as/introspect.oauth2/.well-known/openid-configuration: Not Found

As you can see, this is the problem. Your app can't reach that URL.