I’ve read the manual several times, but where is here the information regarding the authentication?
If the client is only reachable via Internet/through NAT:
Add a new Internet client on the status page.
Download the client installer for the Internet client and send it to the new client. Alternatively, create a user for the new client (in the settings) and send the user the username/password. The user can then download the client installer from the server on the status page and install it.
Select the backup paths you want to backup on the client via command line (“urbackupclientctl add-backupdir –path /” or configure appropriate default directories to backup on the server (see section 8.3.4).
The server will automatically start backups once the client is connected.