OnAuthorization is a TIdOnAuthorization property that represents the event handler signalled when an HTTP client connection needs to provide authentication data as part of an HTTP request. 

VUserName and VPassword are the authentication credentials found in the HTTP Request, and can be updated in the event handler. Handled indicates if the event handler has successfully authenticated using the client account credentials and authentication method. Set Handled to False if any condition in the authentication process is not satisfied. 

Applications must assign a procedure to the event handler to allow responding to the event notification, or to allow the in-process authentication permitted using TIdHTTP.HTTPOptions.