Notification - Post webhook to Splunk Cloud

I am looking to configure the notification settings in our terraform cloud environment.
I ideally would like to post a webhook to our Splunk Cloud instance.
From what I can see this should be possible as I can create a HTTPS endpoint in Splunk. But when I try to do this, it just gives me an error stating it cannot reach the endpoint.

Has anyone managed to get this working or am I totally misunderstanding webhooks.