1. Create a user as normal in the linkstation gui, move that new user to read/write in the access restriction setup for the share.
2. In time machine go to preferences (not 100% sure if preferences is thespecific section), change disk, and when you are selecting which disk totarget (the linkstation) there is an option to use a registereduser. Choose that option and put in the user info that you created on the Linkstation.