The whisper command syntax looks like this, PRIVMSG #jtv :/w {recipient_username} {message}.
ex. if i were to send you a whisper, i woud send this command, PRIVMSG #jtv :/w hurnhu testing., to the Group Chat irc server.
if the response you get when sending the above command is, unrecognized command /w, make sure you have requested the proper CAPs.
CAP REQ :twitch.tv/membership
CAP REQ :twitch.tv/commands
CAP REQ :twitch.tv/tags