I've run into this roadblock also! I was surprised to find I couldn't send confidential Forms results to committee members in a private channel. Security is important, as is automation and ease of use.
Please add the ability to use adaptive cards and Flow Bot in private channels! This is a great use case for automation, but you might have an approval that should only be responded to by a portion of a team (e.g., management).
Hi all, having just tested this, whilst the feature is not available as the flow bot, you can post messages to private channels as a user who is an existing member of said private channel. There are two working actions as follows:
Please note, if you attempt to send a message as the flow bot you will receive an error:
Request to the Bot framework failed with error: '{"error":{"code":"BotNotInConversationRoster","message":"The bot is not part of the conversation roster."}}'.
I really wish FlowBot could post to private channels. As a workaround, I've got a notification flow posting as myself. This means everybody except me gets a notification about a change to a particular file set. I've had to set up a parallel action to have FlowBot send me a chat notification. This really shouldn't be necessary.
This would be a great addition to the toolkit and seems like it would be an easy fix/win for the MSFT team. There are instances in which I need to post to a private channel as I have several different channels set up for different automated solutions that need to be restricted to one or more users.
Updating this Idea to 'Under Review'.