What is the ruling on participating in a weight loss challenge for a sum of money that goes to the winners, with the organizer taking a percentage of the amount?

The majority of jurists hold that if the عوض (stake/prize) comes from the competitors themselves, it is forbidden gambling, unless they introduce a محلل (legalizer) among them who does not contribute any stake. However, if a third party pays the prize, or if there is someone among the competitors who did not pay anything and is entitled to receive the prize if he wins, but owes nothing if he loses, then that is permissible.