ATG Exp - expansion era league

There is a league I would like to join that is ATG Exp. I can't figure out how to select that player set. It doesn't seem to be an option? Closest I can get is ATG Post WWII, but that doesn't let me join.

This league has filled.

It was tucked away here:

scroll through the "Shuffle the Deck" box under ATG8 - it's in there...