Desktop Syncing Clients setup?

What do I put for SERVER in the Desktop Syncing Clients setup?

Your server address, including http:// or https:// (if you’ve set it up with ssl, that is).
If it’s only a local server, the IP address suffices.

If someone else did the setup of the server, you need to ask them. If you rented a server from some hosting provider, they should tell you the address.