Further comments... I've just used almost all of this to integrate to:
https://<my-customer>.freshservice.com/api/v2/tickets/
(note, freshservice.com not freshdesk.com)
We needed to also add a "group_id" field and integer value into the JSON payloads (can be tokenised as per requesterID and _api.key values); we also needed to add a "category" field and value as my customer's FreshService setup has this as a mandatory field with a defined list of acceptable values - you may therefore need to do the same or similar additions for any fields your setup defines as mandatory.
Finally, the JSON path for the created ticket ID from FreshService is (certain for my customer's version) at ticket.id, not merely id.
However, all of these things were very easy to learn from the Integration Logs within LogicMonitor and the results of attempting test alert deliveries, so many thanks @Chris Seiber for providing 98% of the necessary work!