From Onset Developer Wiki
Called when a player was authenticated by Steam. Once this event was called you can use GetPlayerSteamId to get the players SteamId. Steam identity authentication is mandatory. If it fails the player will be kicked.
The player who was verified.
function OnPlayerSteamAuth(player) AddPlayerChat(player, "Your SteamId: "..GetPlayerSteamId(player)) end AddEvent("OnPlayerSteamAuth", OnPlayerSteamAuth)