Hosting an Express Server on Monday's Server Side Code

I plan to have an express application which would be listening to changes on columns before performing a set function.
The Express Application is well hosted on Monday’s server side code. I tried sending a dummy request from my postman to the provided url after deployment and got expected results(logs from Monday’s UI confirmed the setup is fine)

ISSUE
I am getting the error “The provided URL has not returned the requested challenge.“ when I try setting up a webhook automation using the endpoint url provided after deployment.