Ridecamp: [RC] Horse Potty Training (was Stall Matts)

This is a takeoff from the Stall Matt post regarding
training horses to pee in one spot.  
My mare has rarely pee'd when working under saddle in
the two years I've had her.  Perhaps 2-3 times but
that's it.  She is mostly stallbound and perhaps that
could be the cause...but everytime I ride her she
waits until she's back in her stall and sure enough
within minutes she pees in her one special spot. 
Doing it in one primary spot is good when kept in a
stall...but how can I encourage her to do it more
while we are out riding somewhere???
The first time she did it under saddle was when we
went to our first CTR.  It was me that had to go off
in the woods, leading her of course, and she cued off
of me(!!) and she went too :-}  Although this was good
it is not exactly the type of "cue" I want to use with
her to get her to go.  The other times she has done it
was due to another horse or because she couldn't wait
any longer.  Any ideas anyone on how to cue my mare to
pee?
Val + Sania

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