The backup feature in R3 can only write to a directory - it cannot authenticate against an external resource or service.
Traditionally you would mount a network share to a path on your server that R3 has access to - then authentication would be handled by your operating system and R3 just writes to that mounted directory.
Alternatively, you can backup to local via R3 and then run a regular rsync job to copy your files to a remote machine / network share.
I am glad that you are happy with the platform 🙂