Return Username to the Mikrotik

hello, I have enabled the mac authentication in my system, so when the user is connected to the wifi Mikrotik will send a mac address as a username, I am checking if the username is a mac address I am getting the username that belongs to that mac and do the rest of the authentication against the username(not the MAc) so if the user has rights access to the internet I am sending accept but Mikrotik saved the username as a mac address, I tried to reply with the attribute: User-Name="user" but it seems not working are there any attributes or something that I can do that Thanks in Advance