Have you created the application and do you have your webapp ready and listening for webhook callbacks?
The only time you should POST to /1.1/account_activity/webhooks.json is to register your existing webhook listener. What parameters are you sending to the endpoint? Until you’ve registered you webhook, the GET operation will not return a useful response.
Our current sample is in node, but it may help you to understand the principles of how this is intended to work.
Please remember that this API is in beta and that you’ll need to do some of your own work in order to understand how best to use it with your chosen coding environment.