In order to use network shares for a backup, you should click "add" next to the Network shares.
Add the target path and appropriate access credentials: (the domain, the user and the password). This is needed because a backup process has started under System User by default that has no access to the other machines from the network. But the user should have full administrator rights to the destination they would like to back up, otherwise they will get an access denied message and no data will be backed up:
Now the Network shares plugin is available. You can make a necessary selection for the backup: