The only (at least known to me) way to get at the messages in a channel’s chat is to actually JOIN the channel - but be advised, most streamers really dislike random bots sitting in their channels, look at the discussions about faegwent and electricalskateboard, streamers really dislike them and would love to have them banned completely from the platform altogether.
About the whisper: check if your library maybe has a dedicated function for it, what works for my bot is (in raw IRC protocol):
WHISPER username :Text
Or at least it used to work the last time I tried (it’s been a while since then).