So I've spent A LONG time trying to figure out what was going wrong.
From what I can tell there MAY be a bug (unless I'm doing something wrong) with the api.php file.
Example (https://www.whmcs.com/members/includes/api.php)
If you access the file via Internet Explorer you will get a error 403 Access Denied error.
However you access with ANYTHING else, php's cURL, Google Chrome, Firefox it all works. (200 Response)
Normally you would say who cares, but anyone who codes in C# will know that all of C#'s web communication is based on internet explorer or it's base code and will result in the same 403 error, making it impossible for me to connect my app to WHMCS (the only reason I bought WHMCS)
Legit bug or am I stupid?