How to forward alert message to custom attribute of webhook?


I am using OpenDistro 1.9.0 and I am trying to setup alert using webhook. I have a slack webhook like below;

I have setup destination test_slack like the screenshot below;

This will insert sample text alert into the channel my_channel and I can see it. Then I created a monitor and trigger test_trigger and configured the action custom webhook like the screenshot below;

When I click the Send test message link, I am able to see the sample text alert coming in the channel. So the webhook works fine I believe.

My question is, how can i replace the dummy message alert with the original message configured in the Message box in Trigger page, so that I will get the message content in my slack channel?

Thanks in advance.