Watcher Slack action fails to send direct message when configured with the username

[naj-h]

Elasticsearch Version

7.17.11

Installed Plugins

No response

Java Version

bundled

OS Version

N/A

Problem Description

The watcher documentation indicates that in the message.to attribute:

Channel names must start with # and user names must start with @.

While this used to work before, Slack has deprecated passing "username" as a channel value.

I have tested this in a cluster on 7.17.11, the Slack action failed when message.to is filled with a username like @username, the error was channel_not_found. But it worked when providing it with just my member ID.

      "slack": {
        "message": {
          "to": [
            "U98DI989YE9"
          ],

(this is a dummy user id)

This is how I got my own member ID from Slack UI

If we confirm this, the the watcher documentation needs to be updated to match the latest

Steps to Reproduce

• Create a watch with a Slack action
• In message.to field, enter a username prefixed with @
• Save the watch and simulate or execute it
• Notice the failed Slack action

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)