When you prestige on prison there is a crate at '/warp prestige' where you can open your prestige keys which you get every other day and inside the crate you will get a key allowing you to open either a tier one crate, tier two crate and tier 3 crate which have certain rewards inside of them
My man is TRYING for helper! But alas, your response is useless because it doesn't answer his question. Yes, in order to redeem your free prestige I believe you have to be Legend before a staff member redeems it.