Teaching Horses to Urinate on Cue

My cue is to stand with feet apart like a mare preparing to pee, and stretch out my arms

Context: My horses are free to pee and poop any time any where including while I ride. The cue is simply a request to pee now if possible. It is valuable among other things for vet checks, and for avoiding slippery wet floors in trailers.
Here is how I taught my horses to pee outside instead of in their stalls. Other people report success with the same approach. On buggy summer days when they stay in the barn, this gives them a mid-day stretch and saves on bedding. Even though Brandy spends days in her screened in shed, she is included in the routine because she does not like to be left out.
1. Every time I saw them spontaneously pee outside, I rewarded them. (I used Click and Treat, but any reward will do.)
2. When they started watching me while they peed (anticipating a reward) I added a cue. My granddaughter is demonstrating my cue above, but you can use any cue that is convenient for you. I am told that a whistle is the common cue for racehorses to pee.

Bronzz often needs to walk a circle or two while he thinks about it

3. Then when they had been in the barn and I knew they hadn't peed in a while, I led them to the toilet area in the back of the paddock and used my cue. This took some patience. Sometimes we needed to walk a few slow circles at the pee area, pause, and think about it. Leading them thru high grass or weeds seems to stimulate peeing. It can also help to have some non-splash material underfoot like shavings or sand. With practice everyone got more efficient about peeing on cue.
Our Sapphire was the only one who never peed on cue. She was so fastidious she could not stand to splash her legs. Using non-splash bedding in her stall was more important to her than any treat.

Now I can send Shiloh out to pee on her own

While she's busy, I scoop the manure out of her stall.

This time she detours to the water tank for a drink

My horses know that it is always ok to pause whatever I have asked them to do, in order to get a drink.

On buggy days, she wastes no time

Trotting back to a clean stall and a treat

Note: It has been suggested that asking a horse to hold urine could result in a UTI. Horses are NOT asked to hold urine. They remain free to pee any time any where, and they are NOT discouraged from doing so. Teaching a cue can actually be beneficial in helping a horse who is reluctant to urinate in certain places. Race horses and endurance horses are commonly taught to urinate on cue for vet checks and urine samples. No one has reported any adverse effects. My only concern was that treat hound Shiloh might hold urine in order to score a treat, but that has not been the case.
For more useful and fun things you can teach your horse, see Games for Fun and Bonding with Your Horse