Webhooks simulator not posting to slack but does post elsewhere

[confusedm8]

I'm trying to have disputes posted into a slack channel when they arrive. I've add an integration for webhooks and added the incoming webhooks app.

When I use the either url in webhooks emulator, I don't receive anything in slack (the output is successful though). Alternatively when I use a free url from Webhook.site it send the payload succesfully. 

Anyone else have success using webhooks in slack? Any help would be greatly appreciated