It is a Property of Humanoid. So, the player says something.. and the script functions.. plr.Character.Humanoid.CameraOffset = Vector3.new(0,250,0)
plr = Player
Your code to set the CameraOffset in the OP is correct. Are you having trouble defining player?
local player = game.Players.LocalPlayer repeat wait() until player.Character--Wait for the character to load player.Character:WaitForChild("Humanoid").CameraOffset = Vector3.new(0,250,0) --Set the humanoid's CameraOffset after it is created