Sessions on Webgl game

I have a game developed which works with cookies, so it can maintain sessions on our own backend. It works perfectly on Mobile, and also if I run it on WebGL outside of Kongregate.
When on Kong, in preview mode, it fails to retain a session. My best guess is that it's due to CORS. Following this guide (https://medium.com/@alexishevia/using-cors-in-express-cac7e29b005b), I enabled the 'Access-Control-Allow-Credentials: true' from my server (and can see it in Postman), but it requires a change on your side, which of course I cant do... Any suggestions or help you can give me?