First of all, I would recommend building in some kinda verification system, otherwise, they will abuse your game and there is nothing worse than some guys clickbombing your ads (which will get you banned) just to earn a bit of crypto
So, what I would do is build a backend (tracking the count of the in-game currencies, how old is the account, where they come from, IP, etc), do some KYC (know your customer) and then let them request a payout (make sure to build in a threshold, otherwise they will try to request very small amounts), which you can send manually then.