I think there's a very long complicated process to refunding anything digitally. I think your best bet is just cutting your losses and accepting that you accidentally spent 2 dollars lol
When I accidentally bought a character boost in WoW they reverted my account legit in like 5 minutes, I imagine they'd be able to do something similar with OW purchases.
Go to Microsoft store. Go up to an associate and tell them that you purchased a game and that you bought a loot Box by mistake. And then tell them that you accidentally spent $2. They will walk you over to the front desk and have you explain the same situation to another associate that is the customer service they will both stare at you blankly. While they're doing this explain that you spent $2 on a loot box and you want a refund. At this point u can either walk out or keep explaining u want a refund. At the end of the day u spend ur time reading this and gained absolutely nothing out of it except for the fact that you're asking for a $2 refund from a loot box.