It works with the following in an open level options string: ?game=/Game/Blueprints/GameModes/CollectTrialMode.CollectTrialMode_C
Edit: read 's comment for a solution to the following.
The only problem I’ve noticed so far is that it doesn’t load in with the new game mode’s pawn; it keeps the old mode’s pawn. I’d have to spawn and posses the correct one manually.