Calling custom function via REST API from external website

Calling custom function via REST API from external website

I have a custom function set up to accept and return JSON via the REST - this works fine via local REST clients like Postman (using API key or OAuth).

My only issue is that I can't make calls to the endpoint from a web page of an app I'm running. I'm getting the message: No 'Access-Control-Allow-Origin' header is present on the requested resource.

I have added the domain where my page is hosted to the the Trusted Domains section but that hasn't helped.

I'm wondering now if this is supported at all? Will this kind of function call via REST only work if apps are hosted inside Zoho.

Is this supported? If so, is there something I'm missing in configuring our CRM?