Everything you can see in the Finder, you can select as a source or destination. If the server you want to select isn’t visible in the Finder yet, you can mount it by activating the Finder (e.g. by clicking the icon in the Dock), then select the menu Go > Connect to Server (or press Command-K). You’ll then be asked to enter the server address.
After you click Connect, you’ll be asked to enter the login credentials to the server.
If you would like these credentials to be used automatically when connecting to this server in the future, enable the option “Remember this password in my keychain”. The Keychain Access app (installed by default with macOS) will then store the entered credentials. Unless you deactivated the option “Connect volumes automatically” in the sync settings Advanced tab in SyncTime, these credentials will be used automatically when starting a sync and the server volume has been unmounted in the meantime. (The stored credentials are protected with your macOS account password and are not accessible by SyncTime or other apps: when SyncTime starts the sync, it will issue a mount request to macOS which will then look up the stored credentials without disclosing them and connect to the server.)
After you click Connect again, the volume will be mounted and visible in the Finder sidebar or on your Desktop. You can now select it in SyncTime.
Should the username or password change in the future, the login dialog will be displayed again to allow you to enter the new information. Should the server address change, please mount it by following the steps above and then select it again as the new source or destination in the sync settings General tab.