you could make the price to like… 1 robuck and once the game is released, you can change it… or make a gamepass by yourself and test it for free, if it works for a gamepass you made then it should work for the group gamepass too, right?
Studio has a feature that allows you to test purchases without spending any money. You may need to add caching from your end (which is good practice anyway, test purchase or not) which you can accomplish through the use of the prompt finish events.