Sharing Sub Sub Folders

I must be thick because for some reason I can't figure this out. I'm looking for every option in the UI and have come up blank.

I have an AC68U with Melin firmware and I am trying to share a folder further down in the heirachy process but I can't find a way to drill further down within the firmware.

/USB1 (usb stick)
/USB2 (3TB USB drive)
/USB2/Backups (one user has r/w on this directory)
/USB2/Backups/specialfolder (one special user should have r/w on this directory)

I am trying to be...

Sharing Sub Sub Folders