I'd suggest making an account right now (you don't have to necessarily fill it out right now, but if you did, information would carry over to next cycle) and selecting some of the schools you think you'd be interested in applying to and see what kinds of questions they ask- these would be the "real" questions and you can start working on them alongside your PS. This will help tell you what you should include in your PS only, in your secondary only, or in both.
There are many schools that have a secondary application, I think it'd be too many to list them all out. It'd be better to look specifically at the schools you're interested in and go from there. Good luck!