Set up like so: https://support.zendesk.com/entries/382630-hipchat-integration
Add the target here: http://support.YOURDOMAIN.com/settings/extensions#targets
Set up the trigger here: http://support.YOURDOMAIN.com/rules?filter=triggers
Possibly we had to put something ("x") in the trigger action message, even though the instructions above gave the impression you could leave them blank.
We used this for the request URL:
http://api.hipchat.com/v1/rooms/message?auth_token=CHANGE_THIS&room_id=CHANGE_THIS&color=purple&from=ZenDesk&message=%3Ca+href%3D%22http://{{ticket.url}}%22%3E{{ticket.id}}.+{{ticket.title+|+truncate:100}}%3C/a%3E+{{ticket.description+|+truncate:400}}
That shows "1234. The title. The description." with the ticket id and title linked, and both title and description truncated to a max length.
The ticket.description includes an ugly prefix with the sender name and date, but I haven't found a way to lose that without breaking the truncation (as ticket.latest_comment will).