Maybe try what Kayla said, give her a few minutes in the stall to give her time before the crossties. But like Juanmi said, your horse will pee when they need to, and if they’re a mare and theyre in heat... good luck lol! But if they pee while on the crossties, don’t repremand them, just put some shavings over it if you don’t have drainage and then clean it up later.
Put her in her stall for about five minutes before putting her on cross ties. It gives them a couple minutes to use the bathroom or drink
Even do It is not very nice, your horse has to pee when ever necessary. If your Crosstie has drainage, there should be no problem, because with a bucket of water you can remove It.
Hope my answer was helpful.