I find your post confusing… are you:
A) Using the API Connector plugin to call an external API? Which service are you calling? Are there docs for it?
B) Using the API Workflow, set up an endpoint to receive calls from an external server?
C) Both? Calling one Bubble app from another?
Are you able to get it working in Postman?
Normally Bubble looks for key: value pairs in the data, as in:
If you can setup an example on the forum app, it would help to understand the problem and see the data returned. It has API workflows available, can have a look at the other ones there for clues.