How to send direct messages to a user as app in app channel

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

1. Your app needs the bot scope and a bot user
2. Open a direct message channel from your app with the user with conversations.open. You get back a direct message ID.
3. 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.

For those who is still searching for detailed answer:

• First of all you need to make call to this endpoint. You need to make call with bot token and provide into users param value of user you want to send message. Also you need set prevent_creation and return_im to true. Example:

Authorization: Bearer {your_bot_token}
{
  "users": "U12345679",
  "prevent_creation": true,
  "return_im": true
}

• After that you will have your channel id to which you want to send message. Example response:

{
    "ok": true,
    "no_op": true,
    "already_open": true,
    "channel": {
        "id": "D123456789", <-- this is your id
        ...
        "unread_count": 0,
        "unread_count_display": 0,
        "is_open": true,
        "priority": 0
    }
}

• and then with same bot token and user id send message with help of this one

Blockquote I keep getting {"ok":false,"error":"not_in_channel"} do i need to manually add the bot to a channel?? – Abhijeet Bajracharya Feb 4 '20 at 8:04

you need to get scope that allow to send messages like this

There is an alternative way to solve this, which can be more suitable if your app uses a bot to operate with Slack API.

You need to call chat.postMessage API method and specify channel argument equal to the user ID (e.g. U0G9QF9C6) you want to message and as_user argument is true. Important detail - ensure you are using bot access token (learn here how to obtain it).

Example:

curl -X POST "https://slack.com/api/chat.postMessage" -H  "accept: application/json" -d token=BOT_ACCESS_TOKEN -d channel=U0G9QF3C6 -d text=Hello -d as_user=true

In this way, your message will be always sent on behalf (name and icon) of your bot and will be display like a direct message in the app channel (YourAppChannel in the Slack sidebar).

Compared to the approach of @ErikKalkoken you have no need to create a channel in advance and as a result, keep track of its ID (it may be good or bad depending on your needs).