Do they even need to know about it? As far as I know, if someone is playing a game where the wager counts, their wagers will be counted towards the leaderboard, unless a contest specifically need players to sign up for the contest before they make their wagers, but in most cases, you don't need to do anything and gamble normally, and you will still make it into the leaderboard if you are a high roller.

So, I've always had this belief that wagering contests in casinos are not for regular or average gamblers, but they are only for high-rolling whales who have deep pockets and can easily rank at the top of the leaderboard, leaving no chance for average gamblers. That's why, it's useless for someone with a limited bankroll to try and compete in such competitions because they won't be winning anything but will only lose money in the process of trying to reach a certain wagering limit.