Booqable 422 "Url can't be blank" error

Hey there, I am trying to set up a Webhook endpoint on make.com for booqable. See API DOCS

First attached image is the error. I can see it points back to the JSON body text.

I cant see anything wrong with the JSON and I am extremely confused, re typed AND ctrl C+V multiple times now for both the URL for the entire HTTP request and the value of the URL key in the JSON body.

make.com webhook is searching to update data structures.

Where else can I approach to solve this issue?