This is not currently possible. "Webhooks by Zapier" will only respond with the default "success" response because it's intended to be used with any incoming webhook. For that product, there's not special cases for any particular service.
If you're trying to integration with the Slack Events API (something I implemented at Zapier), then I recommend writing your own web service to catch Slack hooks, do business logic, and then send relevant webhooks onto Zapier as needed. This will afford you much more control over the process.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…