AuthServ (or any other service such as NickServ, ChanServ, X, Q, L, etc.) is nothing to do with mIRC but is in fact owned by the network you are connected to. If you register mIRC, it has nothing to do with creating an account on one of those services.
AuthServ is more famous to be present on GameSurge, so I assume you're on that network - if so, information on registering with AuthServ can be found on their website at http://www.gamesurge.net
Hope that helps