Get external_id used in SSO through the API

That looks like the data that’s returned from the regular user route, not the /admin/users/{user_id}.json route. To double check, go to the /admin/users page on your forum. Click on the username of a user you know has logged in through SSO. On that user’s page, add .json to the URL in your browser’s address bar. You should see a json response with a single_sign_on_record property.