Pass service domain to api when adding a new service

We need to pass the domain to api when adding a service so that api can
link the domain of the service with a letter brand.

tests/app/notify_client/test_service_api_client.py

@@ -74,6 +74,7 @@ def test_client_creates_service_with_correct_data(
         True,
         fake_uuid,
         'test@example.com',
+        'nhs.uk'
     )
     mock_post.assert_called_once_with(
         '/service',
@@ -89,6 +90,7 @@ def test_client_creates_service_with_correct_data(
             restricted=True,
             user_id=fake_uuid,
             email_from='test@example.com',
+            service_domain='nhs.uk'
         ),
     )