I have created a simple Slack App app where the only purpose is to send a message to a channel. I understand that there is the conversations.list API to list all public channels to get the correct ID.
However, as a first step, I just want to send the message to the app channel itself. If I use the D... ID it works as expected. No invite by the channel is needed. But how do I get this ID? conversations.list only returns publich channels, but no the app channel itself.
In Slack, there is no such thing as an app's channel. There is a DM channel between every user and your app/bot. In these terms, to send a DM message from your app/bot to the user, you need to know ID of this user and specify it as a channel argument of the postMessage API request.