To ensure that target server is ready to consume notifications, we require you to implement the initial
Having subscription with
https://example.org/hook, during activation we will make
POST https://example.com/hook X-Hook-Secret: 4jn8fs9011jj8
We expect your server to respond within the time of 20 seconds with HTTP
200 response containing
X-Hook-Secret header with same value.
Response 200 X-Hook-Secret: 4jn8fs9011jj8