xMatters webhook in a workflow transition

[JDave]

I have defined a webhook - for xMatters - and I have added it to the a post function on an incident workflow - on the create transition.    The post function that I have added is "Send transition to webhook: xMatters      From the incident - the Engage with xMatters functionality exists and works fine.  

I want to fire an xMatters notification on incident creation - Is the post function not where I need to do this?     The Webhook is setup as below - the project is selected, issue type is Incident.