You would connect to it just like you currently do for the main Twitch servers. Replace “irc.twitch.tv” (if that’s what you are using) with “199.9.253.119”.
Whispers are intentionally made difficult to be used over IRC since they will be moving off that platform at some point in the future.
For casual chatting I’d recommend looking at the beta versions of Chatty which will work with whispers.