Hi,
if you are using ZFS, create a new file system with zfs create
and set the quota
property accordingly. If you are not using ZFS and the whole disk is already used by a single file system (default, if you don't opt to keep free space on the disk during installation), I think you need to set up the quota for the backup
user that exists by default (but I haven't tested this!). In all cases, just add a datastore with a path on the root file system (for ZFS with the path pointing to your newly created file system).