I have succeeded in creating and testing the Custom Connector and it works fine. The challenge I am having now is, I am unable to call the API from inside the powerapp. I want to bind the API to the drop-down field such that the data automatically populates the drop-down field OnSelect or OnChange.
I am looking for exactly the same functionality. I looked at a similar thread that I am not sure would help you -
For me this thread did not work as the response I get from the custom API connector querying a graph API saves the outcome in a collection with arrays. I am figuring out how I could retrieve the result from a collection array and have it binded to a dropdown list.
Is there some error message when you call your API within your app?
Could you please share a bit more about your issue?
Further, could you please share more details about the Custom API formula you used within your app?
I agree with @alzia's thought almost. Please check if you have specified proper response schema for the specific Operation within your custom connector.
In addition, please also check if you have specified proper response type within the Response part of your OpenAPI definition file.
You could consider take a try to test your API using Postman tool, and then get the response body of the '200 OK' response, after that, copy that response body and paste it into the Response schema of the 200 response for the specific Operation in your custom connector.