Do HubSpot APIs support CORS / AJAX requests?

At this time, HubSpot APIs do not support support cross-origin (CORS) AJAX requests. Making the request client-side using JavaScript would expose any authentication you're using for the request. In order to use JavaScript/AJAX, you would need to make the request (excluding any authentication) to an external server that could then add the needed authentication and make requests to HubSpot's APIs server-side.

For example, if you wanted to submit form data to the Forms API using AJAX, you would need to send that request to an external server, process it server-side, and post to HubSpot's endpoint from that external server.