Personally, If I was to donate, I'd do it straight to the org of preference.
That would be satisfying yourself and that's okay since you are the one in control of the funds. People who have a specific feel on a charity should be considered and the possible emotional connection with a certain charity. Maybe some traumatic event that led them to donate or something.
An option for casinos could be adding the choice for winners to donate a percentage of their winnings. It coul be a direct question every time they win or a pre-set choice from the user's profile.
Maybe the casino can input it when withdrawing? Or casinos collect more fee and a certain portion of that could be for donation or something. Sometimes when I withdraw in an ATM, there's a donation page if you would donate a few cents for a charity or something.