Hetzner Storage Boxes are a cheap and useful way to store any data or backups. They come with support for RSync/SSH, FTP/SFTP, WebDav and Samba/CIFS. Normally they have their services well documented, however they never explained how you can mount a CIFS share from a sub-user account.
The hostname of your storage box has to referr to your main username, but the actual user and directory/share-name has to be your sub-user:
Linux the command will be like this:
mount.cifs -o user=<sub-username>,pass=<sub-password> //<main-username>.your-storagebox.de/<sub-username> /SOME/TARGET/PATH
For further informations on how to access your storage box see the documentation on docs.hetzner.com.