When you add a service, you are exposing an API endpoint owned by EclecticLabs. In order to do this, the Trafficker needs to know how to retrieve the API when a client is calling it. This can be achieved either by specifying a custom DNS address or a specific URL path.
Once you have decided what form of request to which you would like to respond, the Trafficker needs to know the final target URL where the requests will be proxied.