When an OAuth2 client do a request to get a new Access Token, a Web-Page is shown in a web-browser to Allow this connection and login with an User and Password.
The HTML page is included by default in Server component, but this code can be customized using OnAuth2BeforeDispatchPage event.
If you customize your HTML with a completely new HTML code, at least you must maintain the form where the Username and password are sent:
The id parameter, which is hidden, must maintain the same value of the original form to allow server identify the request.