Issue with setting up at the subscription to pull request events step.

[jamesray1]

This didn't work (note I replaced the secret key, below is not it):

curl -u "diamond-drops-clabot" -i https://api.github.com/hub -F "hub.mode=subscribe" -F "hub.topic=https://github.com/Drops-of-Diamond/diamond_drops/events/pull_request" -F "hub.callback=https://peaceful-sierra-37396.herokuapp.com/notify" -F "hub.secret=somesecretkey"
Enter host password for user 'diamond-drops-clabot':
HTTP/1.1 100 Continue

HTTP/1.1 422 Unprocessable Entity
Date: Fri, 20 Apr 2018 02:37:09 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 212
Server: GitHub.com
Status: 422 Unprocessable Entity
X-RateLimit-Limit: 5000
X-RateLimit-Remaining: 4995
X-RateLimit-Reset: 1524192645
X-GitHub-Media-Type: github.v3; format=json
Strict-Transport-Security: max-age=31536000; includeSubdomains; preload
X-Frame-Options: deny
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
Referrer-Policy: origin-when-cross-origin, strict-origin-when-cross-origin
Content-Security-Policy: default-src 'none'
X-Runtime-rack: 0.047518
X-GitHub-Request-Id: E5C8:34DD:3ABF88:49366B:5AD95254

{
  "message": "No repository found for hub.topic: \"https://github.com/Drops-of-Diamond/diamond_drops/events/pull_request\"",
  "documentation_url": "https://developer.github.com/v3/repos/hooks/#pubsubhubbub"
}

I have set up the app and got a token. I set up a HerokuBeta service in the above repo, https://github.com/Drops-of-Diamond/diamond_drops, but that didn't help, I got the same error as above. Do I need to set up a webhook or what do I do? AIUI PSHB is used as an alternative API to the JSON HTTP API.