ALWAYS let horses go in their stalls to urinate after cooling down from work and before grooming or bathing. It honors their needs. IMPORTANT HABIT! Horses do look forward to it.
On the track they whistle when they go. Then, they whistle to make them go when it is useful. Handy. There is a specific whistle they use. Ask an old-time horseman. He or she can probably teach you that standard whistle.
Here's more on why they need to go stop by their stalls.