Adobe campaign classic V7 to Graphql connection

There is a use case to call a Graphql based API provided by external vendor and we need to call that API from ACC, use the response to define journeys.

We are doing a POST call with all possible headers, connection works and we receive a 200 response code but the response body comes back as HTML response. We are able to test this successfully from Postman.

Anyone have insights if there are compatibility issues and ACC doesn't support a Graphql connection?

Thanks.