How to provide custom webhook response in flows

I have a postman flow that I want to trigger with webhook.

for me it is a Facebook webhook that will be called every time there is a new post in feed.

problem is that to configure webhook, I need to verify security by configuring answer to a challange.

currently when I configure webhook in postman flow, Webhook response can only be default or empty. I need to be able to grab a challange id from webhook request and send back to facebook