How do you customize payload when using webhooks?

Hello developers all over the world!

I’m trying to connect discourse to slack using webhooks.

I’ve done most of things but only one thing is left.

when you create a new topic in discourse, you send payload which has a lot of parameters to slack.

however, slack only get ‘text’ parameter. so I want to customize payload when using webhooks.

how do I change payload when using webhooks in discourse?

thanks for reading!