In Kibana go to Alerts and Actions. Click Create Alert.
Open the dev tools in your browser. Typically F12.
Go to the Network tab.
Now fill in your alert you want to create for a test. When you save it you will see a new entry in the developer tool. I think it's called alert. If you click on that you can see the API call that was made to create that alert.
Without official documentation and support of this API I can't really steer you in any other direction.