@Chris Seiber thanks for posting this. I have found a couple of things when setting this up and thought I'd share (and ask for feedback):
1. To properly format the message in the ticket you need to use HTML. In other words you need to use <br> instead of \n for a line break.
2. The JSON path you have doesn't seem valid. I was able to use $.id to get the ticket ID... however there is an issue with that (see below).
When I get the ##EXTERNALTICKETID## it is coming back like this:
Slack Integration : -1, Freshdesk - Create and Update Tickets : 20018
Where the first part before the colon are the display names of all integrations used on the alert (note the first one is for Slack and the second is for Freshdesk). If I only had the Freshdesk integration configured, it would likely say "Freshdesk - Create and Update Tickets". The part after the colon is the actual ticket number. Do you have any thoughts on how to strip off the first part? I know you support regular expressions but then that doesn't seem to be usable at teh same time as JSON for the ##EXTERNALTICKETID## mapping. Is this a bug in your software that is including this in the property to begin with given I don't see how we could possibly get those values (the integration labels) from the JSON response itself (I don't see them when I do a test).