I was trying to incorporate SSO in Freshdesk with WHMCS, but because WHMCS requires passing a query parameter = access_token instead of using the Authorization header, this is what you need to do to make it work.

1. Reveal Authorization Header in .htaccess

2. Use your own oauth-intermediary.php where you take the header and send the query parameter that WHMCS is expecting

3. User your new https://www.yousite.com/oauth-intermediary.php as the new userinfo.php URL when requested