How to send trigger action to specific mattermost (Slack) channel

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser):

OpenSearch Dashboard: 1.3.13
Platform: k8s

Describe the issue:

I want to send the alert to specific channel in mattermost.
Using the webhook, notification goes to “testalerts” channel but using same webhook I want to send notifications to “prodalerts” channel

Configuration:

Configuration is done from OpenSearch Dashboard GUI

Relevant Logs or Screenshots:

I tired to put key/value pair with custom plugin option. It fails.

If I configure using Slack with webhook URL, I am getting error as follows when I test the alert = java.io.IOException: Failed: HttpResponseProxy{HTTP/1.1 400 Bad Request

I tried to use Mustache template in the “Message” section but it is not sending the alert in the channel where I would like to route.