In the 'Advanced Options' menu on the 'Create A New Tournament' page, look for the 'Game Type' option.
From there, you can choose from a number of book openings by choosing 'Book Opening.'
https://support.chess.com/customer/en/portal/articles/1413077-thematic-games?b_id=12321
Or, if you want to make up your own, made-up starting position, choose the 'Custom FEN' option. I've personally never created a tournament using this option, but it looks like you'll be able to figure it out by referring to the second half of the page found at the following link:
https://support.chess.com/customer/portal/articles/1444914-what-is-pgn-fen-
I don't see the option to create theameti tournaments??