Unable to ping Authorize.Net webhook

Question

I just started building a webhook following the documentation. I created a sample web api application and published to IIS with a valid url accessible from the internet.

I created a webhook on out authorize.net sandbox account and set the url. I am able to call the POST api method from inside the postman. But when I try to ping the webhook I get error as follow:

https://apitest.authorize.net/rest/v1/webhooks/70884522-6fe8-4e72-9c7e-15bf2b30f778/pings

  {
    "status": 500,
    "reason": "PING_FAILED",
    "message": "The ping operation failed with the status code 403",
    "correlationId": "5d136ce8-406f-442e-9118-fdde36946d87",
    "details": [
    {
    "message": "Error occured in connecting to the endpoint:http://staging4.dancecompgenie.com/api/Values/Post "
    }
    ]
    }

Here is the url on my server.

http://staging4.dancecompgenie.com/api/Values/Post

I am able to successfully call this url in postman and get the result.

I appreciate if you can help me solve the problem.