Activate webhook subscription.

To ensure that the target server is ready to consume notifications, we require you to implement the initial

Having subscription with callbackUrl:, during activation we will make POST
request with X-Hook-Secret header:

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 the same value.

Response 200
X-Hook-Secret: 4jn8fs9011jj8
Click Try It! to start a request and see the response here!