It looks like it is not possible to specify a starting position.

The combo is there, but empty.

Any chance we can have some famous position in the list of starting position or allow user to specify the FEN. For instance, for the Keller, it would be:
B:W24,26,29,33,34,36,37,38,39,40,41,42,43,44,45,46,47,48,49:B2,3,4,5,6,7,8,9,11,12,13,14,15,17,18,20,21,22,23

I was able to do it in a game, but not in creating the tournament.

It is not a priority but would be nice to have for people studying some specific position by participating in tournaments under those themes.

In principle this option exists for tournaments, but a list of opening positions hasn't been created yet. At the moment we only have the Russian / Brazilian FMJD opening tables for this purpose.

Maybe we can collect a list of opening positions in this forum topic. Surely there are people with good opening knowledge around, who can suggest some well known and interesting positions. So shoot if you have more suggestions!

I also like the idea to add an option to create tournaments with a custom FEN as starting position.