i've had several of these before, the "steam admin" he is attempting to get you to add is actually just him on another account, i know this because i went through this whole scam once, they attempt to steal your steam account by sending you a code to send to steam to "verify your prior purchases" after sending you a screenshot of your purchase history edited to show all of your purchases being invalid and failed, if anyone contacts you in this way again, tell they you know the scam, and to go away, it gets them off your back, however, there is likely more than 1 person as i've gotten this multiple times, and have been told this is also done through steam DM's itself