Hi Tina, 

This is actually something that I dug into very recently as I was trying to do the exact same thing.

It’s not possible without some intervention on the Slack side of things. Thankfully though, it’s a pretty easy task that you’ll have to perform.

The Customer.io Slack documentation is where all this is answered in case you haven't stumbled across it yet. It might come in handy in the future, particularly this portion:

Posting to a private channel
If you want to post messages to a private channel, you must invite your bot to a channel with the /invite @Customer.io command.

That should solve what you need - adding the bot to that channel will let it post in there and you’ll be set. 

Thanks for raising this, as I think it’s a good one to discuss in this forum!

Cheers,

Ami