Form logic for calling a REST API to populate a single field

1. What is the issue? Please be detailed.
In ODK Collect, I am attempting to populate a single field in an XLSForm form by calling a REST API when the user accesses the widget. I am lost at how the XLSForm can be defined to trigger this action.

2. What steps can we take to reproduce this issue?

3. What have you tried to fix the issue?

4. Upload any test forms or screenshots below.

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.