This endpoint takes in a callback URL, and a list of event types that you would like to receive updates for. During this request, HCSS will make a GET request to your callback URL with a query parameter called "challenge". Your endpoint must return a 200 OK with the challenge in the response. Once your callback URL has successfully responded to the challenge, you will begin to receive POST requests to the same URL. Information regarding the shape of these POST requests can be seen in the "Callbacks" section below.