Changing the Start Boxes?

How do you change the start boxes in the current version? The map Valiant Saltscape shouldn't be played up-to-down, as evident by the high-metal spots at the center line. But since that's the default, how do I change it for my own matches? None of the old split commands that I remember seem to work and none of the Adv. settings seem to do anything, unless I'm missing something.

This can't have been removed, right?