Out of the box June doesn't support sending notifications into private Slack channels.
This is due to a limitation on Slack's side that doesn't allow us to see all the private channels in your company.
If you still want to receive Slack notifications in a private channel though there's a little hack we can share with you.
To receive Slack notifications in a private channel there are 4 steps you should follow:
Create a public channel for the notification you want to receive
Create the event notification that you want to receive in that private channel and enable it!
Wait to receive the first notification from June - So we make sure everything is working fine
Make the channel private in Slack
You'll now keep receiving those Slack notifications even if the channel is private.
Below you can find a screenshot of how your Slack's channel timeline should look like after following these steps: