NewGame screen, bottom left hand corner, "set up your own board". It's easy enough to work out once you get the relevant screen, and these games are always shown with the set-piece symbol and unrated. It's not in advanced options.
I have a few games going here, but I'm disappointed that the castling doesn't work at all, even though the page says it's available. I'll stick with the chess.com system which works for castling, even though it has another problem.