Training dogs *where* to potty in the yard.

I have two, 11-year-old rescued greyhounds who are walked once a day and hang out indoors other than when they use their dog run (small, fenced enclosure) for potty business that isn't done on their daily walk. They know the command "go potty".

We just got a new fence for the whole yard and I would like to allow them access to the full yard for play, but still have them potty (poop, at least) in the dog run, which is connected to the rest of the yard with a gate that I can prop open.

Other than constantly walking them from the main yard into the dog run and saying, "go potty," (and praising/treating when they do, like when I first housetrained them) what are my other tactics for training them to potty in the dog run when they are playing in the main yard?

Thank you for any advice!