Once the user has been logged in, they must complete the protocol workflow so they can ultimately be logged into the client. To facilitate this, the login page is passed a returnUrl query parameter which refers to the URL the prior request came from. This URL is, in essence, the same authorization endpoint to which the client made the original authorize request.
In the request to your login page where it logs the user in with a call to SignInAsync, it would then simply use the returnUrl to redirect the response back. This will cause the browser to re-issue the original authorize request from the client allowing your IdentityServer to complete the protocol work. An example of this redirect can be seen in the local login topic.
Beware open-redirect attacks via the returnUrl parameter. You should validate that the returnUrl refers to well-known location. Either use the Url.IsLocalUrl helper from ASP.NET Core, or use the interaction service from Duende IdentityServer for APIs to validate the returnUrl parameter.
Keep in mind that this returnUrl is state that needs to be maintained during the user’s login workflow. If your workflow involves page post-backs, redirecting the user to an external login provider, or just sending the user through a custom workflow, then this value must be preserved across all of those page transitions.