I haven't been able to track down any information on my own about this, so no one must be having an issue.
I tried to create a dispatch rule to take a ticket with a subject starting with CHN and launch a webhook POST to create a change request.
The webhook doesn't appear to run at all, I can set any other number of options to modify the ticket and everything works, just not anything as a webhook.
I have created a POST using the link below
I have tried the following
* No authentication, API Auth, User Auth
* Different trigger options (Sender, Subject)
* Changed rule to set a different custom field (source) and a Supervisor Rule to generate the Change
Sorry for the inconvenience caused.
Please find below the implementation for the your use case.
Hope that was helpful, feel free to reach out for further support.