Hi AllanHeo - Its not Darren T sorry - its Darren B 🙂 I've chatted to you on the old Slack groups previously
I did some more testing what what I was after and rather than fixing the result to two static URLs, I decided to pass in the URL via the API call as a callback URL parameter: canvas_entry_properties.${callback_url}
However, I found that there is some restriction on what you can put in the Webhook URL field. It seems to always want a HTTP scheme as a minimum e.g.
https://{{canvas_entry_properties.${callback_url}}} (Saves OK)
{{canvas_entry_properties.${callback_url}}} (Throws error on Save)
Cheers
Darren