Sending to a private channel as a bot can only be achieved if the bot (Introist) is a member in the channel.
Here's how to do that rather simply:
- Go to the private channel you would like to use
- Post a message "@introist"
- Slack should ask if you would like to invite them to the channel
- Click "Invite"
- Introist is now a member of that Slack channel and you can use the channel as message target while using a bot as a sender!
- Finally, if you like to clean things up, you can remove the message "@introist" that you just posted