Sending message to different channel via slack webhook fails - webhooks

Question

I'm not sure if I am understanding the way how to use webhooks quite right, but:
I want to send messages in different channels or to different users (not at once), without being involved in that conversation.
My problem: I can only create webhooks for specific users, so I end up having a unique URL for every user / channel?
If I use something like the "channel" parameter or even the "setReceipent" method from the library I use, it doensn't have any effect and only the channel / user the webhook was created for, receives the message.
Do I need to use API access or can I accomplish my needs using webhooks?

Answer 1

In general incoming webhooks are fixed to the configured channel. So if you want to send messages to users and/or multiple channels you need to create multiple webhooks or send messages though the API (e.g. chat.PostMessage).
However, there is a another way to create webhooks, that allows you to send messages to every channel with the same webhook by adding a channel override property ('channel') to your message. Its how incoming webhooks used to work in the past and part of legacy custom integrations.
To create such a webhook you need to install an app called "Incoming webhooks" from the Slack App Directory (app is made by the Slack team).
Syntax:
POST https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX
Content-type: application/json
{
"text": "Hello, world.",
"channel": "U12345678"
}
Where U12345678 is the Slack ID of the user you want to send a direct message to.
Note that the channel property is optional and the message will be send to the default channel if omitted.
See here fore the full documentation.

Answer 2

Hooks only allow you to send to the channel defined in the hook.
If you want to send to any channel you need to create a bot user who can post to any channel. To create a bot user you need to do the following:
Add an API app Once done create a bot user (or create and delete a webhook )
Create a bot user or create and delete a webhook (which will create the bot user for you)
Add chat:write and possibly chat:write.public to the OAuth & Permissions of the API App.
Grab the Bot User OAuth Access Token it should start xoxb
Post to https://slack.com/api/chat.postMessage
eg
curl -X POST \
-H 'Authorization: Bearer xoxb-###-###-***' \
-H 'Content-type: application/json' \
--data '{"channel": "#general","text":"Hello, World!"}' \
https://slack.com/api/chat.postMessage
https://api.slack.com/messaging/sending#publishing provides some details