You realize mIRC is not used solely for twitch, and that how twitch operates is generally not related to mIRC? Ask somewhere twitch related how to obtain that information. Then you can ask specifically how to implement it in mIRC.
Where does "<name> just subscribed!" come from then? Is that a private message? You can't just assume people will know these things, how twitch operates has nothing to do with IRC in general. There's no concept of a subscriber.