Let’s go DM!!
Also, it appears the redirect URI for this website was incorrectly configured for the OAuth app registration with Microsoft. It needs to include the full path, just not the query string. It should be set to https://www.ksiforums.org/applications/core/interface/microsoft/auth.php. Setting it to https://www.ksiforums.org/ or https://www.ksiforums.org/applications/core/interface/microsoft will not work.
As a result, it is impossible to sign-up with microsoft.