I'm only just getting started, but my Flic Hub is successfully sending off HTTPS-requests for button events. I'm sending GET/POST/PUT/DELETE to my own server and it's working fine, and very quickly.
Have you confirmed the Hub has internet access? Could you try with another URL endpoint to verify the requests come through?
The only issue I have right now is that there's nothing useful included in the request. I can manually add headers and body-data, but it would make life much easier if the button-id was included in all requests per default.