Unfortunately, you can not variablize the source URL.
If you would like to see this feature in a future release I would suggest you vote for Option Lists URL and Header fields should support variables
That said I’m not sure I’m seeing the use case in this scenario. Why do you need the return in an option list?
Would it not be better to make a custom option to take in an app ID and use that custom option value in a task to make the API? You could then output the return so it can be viewed or used in another task.