Private messages were removed from Twitch Complete.
If you refer to Whispers instead, the general advice is to not bother with Whispers as bots do not have reliable delivery rates with them due to abuse by spammers
Whispers are send and Received over IRC/Chat/TMI
They are revived as a message for Command WHISPER and sent via .w target message here
What Alca wrote in the thread you have linked still applies. But Bots generally have issues with delivery success rates