If I understand your question correctly, you want to send direct messages to users in the app channel instead of the standard slackbot
channel.
In order to do that you need to
- Your app needs the
bot
scope and a bot user - Open a direct message channel from your app with the user with
conversations.open
. You get back a direct message ID. - Send a message with
chat.postMessage
to the the direct message channel ID
Make sure to use your bot access token (not the user access token) from your Slack app.
The bot
scope gives you all permissions needed to open and send DMs to users from your bot channel. No other scopes are required.
You can also use the new conversations
methods, which work for all kind of channel types to do the same.
See also this question on the same topic.