As for cues, it might be a verbal out or asking with your seat. That’s how my SS horses have been trained–no tapping on the shoulder with a toe.
But, as some other posters have mentioned, you might not want to ask for the behavior or you might find it offered at the halt. NOT what you want. Many SS horses understand that parking out means be still/don’t move. I’ve been very careful about not letting my SS convert to sporthorseland park out, because he used to do it with the settling of my seat. Slick in the show ring, but bad news in the dressage ring!
Although, I did inadvertantly resort to it just a few weeks ago. Showing off the trailor, which he isn’t used to, at a busy show. He was so amped after our ride that he was becoming quite obnoxious and wouldn’t stand still. Embarassing, to say the least. I got so exasperated that I told him “OUT” without thinking about what I was doing… it was like a magic pill. All of a sudden, he was standing stone still, all parked out and perfectly composed. It was actually quite funny–like he was saying, “Oh, so you want me to stand still… why didn’t you just ask??”