There is no way to create a form that can populate a field from a REST API using just ODK Collect.
The best way to do this would be to create an app (or find an existing one that does the job for you) that can make the API request and return the value to Collect using the External app string widget. However, keep in mind that a flow like this could prevent the form from working properly when the enumerator's device has no internet connection.