At the start of a game. Be able to make your first move as white. And when someone accepts your game it will be ready for them to make the move as black.

And if you are black you could maybe do it by guessing what your unknown opponent of white might do. So if they make the right move them your black piece will move.

I know it works in tournament play on first move as black. I did it before by guessing they would play 1.e4 because that is always this opponents first move.

As the game starts you can move as white. Before the game was created there is no possibility to put on a conditional move.

Your suggestion is a tournament with a fixed openig, say e4. But then you can't put in a black conditional.

Could you suggest a way to tack a move to a uncreated game?