is there another step I'm meant to do via the UI? I don't see anywhere to add a service, and my app doesn't show up in the service map

@User you don't need to add anything from the UI. The service name provided as env variable while running would be taken automatically. The service map might not be showing your service coz it is unrelated to other demo services running. So, it must be alone somewhere not visible in current zoom level at service map. You should be able to see your service in dropdown options in service map