Sync Clients must specify how to reach the Sync Server. In a test setting, both my be running on the same machine. In this case, you can use ws://127.0.0.1 as the destination; 127.0.0.1 is the IP address of localhost. If it's separate machines, you need to exchange 127.0.0.1 with an reachable IP address of the server, or, some valid DNS name.