Open up the Order History and select the app or game you want a refund.
Also, to be completely honest, it sounds like you're pulling this all out of your ass.
And no character restrictions like the demo would or a sports game as an example, no team restrictions.
Their hands are most likely tied as they could jeopardize their own job if they give you the refund.
It's a nuisance, for sure, and in no way acceptable, but I'm pretty sure it's a Halo Wars 2 problem, not a store-level problem.
If the game or game purchase is broken and doesn't work, doesn't deliver what it promised, or simply doesn't appeal to the purchaser, Microsoft provides methods with which players can seek a refund or cancellation.