Client portal - improve documentation #732

Author: maartenba

src/content/docs/identityserver/ui/portal.md

@@ -9,12 +9,83 @@ redirect_from:
   - /identityserver/v7/ui/portal/
 ---
 
+You can create a client application portal within your IdentityServer host that contains links to client applications
+that are configured with an `InitiateLoginUri`. The `InitiateLoginUri` URI property is optional, and can be used to
+[enable identity-provider initiated sign-in](https://openid.net/specs/openid-connect-core-1_0.html#ThirdPartyInitiatedLogin).
+
+Your IdentityServer host can check for clients with this property, and render links to those applications for the
+currently authenticated user.  Doing so gives the user a client application portal that lets them start using each
+application, where navigating to  an application link starts an OpenID Connect challenge with the application. 
+
+This creates a curious pattern, where the user follows a link from the portal page in the IdentityServer host to
+an external application only to have that application immediately redirect back to the IdentityServer host's
+`/connect/authorize` endpoint. However, if the user has logged in and created a session at the IdentityServer host,
+they will get a single sign on experience as they navigate to the various applications in the portal.
+
 :::tip
-**Added in Duende IdentityServer 6.3**
+The [Entity Framework Core project template](/identityserver/overview/packaging/#templates) comes with an example
+`~/Portal.cshtml` Razor Page that implements this functionality.
 :::
 
-You can create a client application portal within your IdentityServer host that contains links to client applications that are configured with an `InitiateLoginUri`. `InitiateLoginUri` is an optional URI that can be used to [initiate login](https://openid.net/specs/openid-connect-core-1_0.html#thirdpartyinitiatedlogin). Your IdentityServer host can check for clients with this property and render links to those applications. 
+## Third-Party Initiated Login
+
+The [OpenID Connect Core 1.0 specification](https://openid.net/specs/openid-connect-core-1_0.html#ThirdPartyInitiatedLogin)
+describes several query string parameters that can be passed from the identity provider to the client application:
+
+* `iss` - a URL (using the https scheme) that identifies the issuer
+* `login_hint` - a hint about the end user to be authenticated
+* `target_link_uri` - URL that the client application is requested to redirect to after authentication
+
+These query string parameters are not included in the template IdentityServer client application portal, but you can add
+them to your implementation when desired.
+
+## Implement Identity-Provider Initiated Sign-In
+
+To support identity-provider initiated sign-in, client applications must:
+
+1. Be registered in IdentityServer with the `InitiateLoginUri` property set to a URL in the client application.
+2. Implement an endpoint at that URL which triggers an OpenID Connect authentication challenge.
+
+### Configuring The Client In IdentityServer
+
+In your IdentityServer client configuration, set the `InitiateLoginUri` property:
+
+```csharp {7}
+// IdentityServer Configuration
+// ...
+new Client
+{
+    ClientId = "myclient",
+    // ... existing config ...
+    InitiateLoginUri = "https://example.com/signin-idp"
+}
+```
+
+### Implementing The Endpoint In The Client Application
+
+In your ASP.NET Core client application, implement the endpoint referenced by `InitiateLoginUri`.
+This endpoint should trigger the OpenID Connect authentication challenge.
+
+Here's an example ASP.NET Core endpoint that redirects the user to IdentityServer for authorization.
+When the user is already authenticated, the user is redirected to the application root.
+
+```csharp
+// Program.cs
+app.MapGet("/signin-idp", async (HttpContext http) =>
+{
+    if (http.User.Identity is { IsAuthenticated: false })
+    {
+        var returnUrl = "https://example.com/";
+        
+        return Results.Challenge(
+            new AuthenticationProperties { RedirectUri = returnUrl });
+    }
+    
+    return Results.Redirect("/");
+});
+```
 
-Those links are just links to pages within your client applications that will start an OIDC challenge when the user follows them. This creates a curious pattern, where the user follows a link from the portal page in the IdentityServer host to an external application only to have that application immediately redirect back to the IdentityServer host's `/connect/authorize` endpoint. However, if the user has logged in and created a session at the IdentityServer host, they will get a single sign on experience as they navigate to the various applications in the portal.
+For the challenge to work, an OpenID Connect schema must be configured in your client application.
+When multiple OpenID Connect schemas are registered, you can also use the `Results.Challenge()` overload that allows
+you to target a specific schema.
 
-The quickstart UI contains an example of such a portal in the `~/portal` razor page.