You can't use a local URL. This is because a webhook request is a POST request
that is made from our server. Therefore, the only URLs that we can make these
requests to would be URLs that are publicly available.

For testing purposes, I would recommend using RequestBin. You can create a
new bin and paste the URL for that bin in the webhook URL field of your
Instamojo link. This will give you a link that is accessible by our server
and you can inspect the POST requests to this link by appending ? 
inspect at the end of the URL.

request bin new url : https://beeceptor.com/