Moneris Checkout (MCO) - Preload Request POST CORS error

I'm attempting to setup a Moneris Checkout page.  I'm using the documentation and trying to run the preload request via an AJAX POST, however I am getting a CORS error when I do this:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at (Reason: CORS header ‘Access-Control-Allow-Origin’ missing).

I've added Header set Access-Control-Allow-Origin "*" to my .htaccess file but the error still persists.  Any idea why this is happening?  Is this coming from my web server or the test server?

Thank you for any insight you can provide!