I've been trying to get this to work as well.
Not sure if you had any luck.
I have created a HEC instance and tested it.
I created a webhook, but unsure of the URL to correctly add. I simply get HTTP 500 status when I test the aruba webhook.
I'm sure there is a better way to setup the URL on the aruba side so it correctly sends data to inject.
I did get it to work. Much trial and effort as it was my first experience at HEC and webhooks.
This was the correct URL I figured for the Aruba central to send to my HEC.
https://<HEC URL>:8088/services/collector/raw?token=<your token>