So, my solution as of now is to create my own pseduo userinfo php file in my whmcs directory.
The first thing I need to do is to reveal the authorization header via .htaccess
CGIPassAuth on
And then create oauth-intermediary.php file that takes the Bearer userinfo request and does a POST request with access_token query string
<?php
//OLD USERINFO https://www.yoursite.com/oauth/userinfo.php
//NEW USERINFO https://www.yoursite.com/oauth-intermediary.php
header('Content-Type: application/json');
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL,"https://www.yoursite.com/oauth/userinfo.php?access_token=".str_replace("Bearer ","",getallheaders()["Authorization"]));
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$server_output = curl_exec($ch);
echo $server_output;
curl_close ($ch);