I found some of my notes on using the http target:

HTTP target

Title: Update Zendesk Ticket

Url: https://domain.zendesk.com/api/v2/tickets/{{ticket.id}}.json

Method: PUT

Content Type: JSON

Basic Authentication: Enabled (use an agents login with "/token" on the end and then use the API key as the password)

Trigger

Set any conditions you need

Perform these actions:

Notifications: Notify target Update Zendesk Ticket

JSON body:

{"ticket": {"recipient": "accounts@support.domain.com", "custom_fields" : [{"id":29710328, "value": true}]}}

Some notes:

1) In this example I not only set the Recipient value but also the value of another custom field. Just to illustrate the power

2) The email address MUST be one that is verified as a Zendesk support email address. if not then your default support address will be used.

3) This update does not happen immediately. Therefore if you need to update the Recipient before you send an email notification (perhaps when a ticket is created) you will need to do a little more trickery first.

4) Updating the Recipient does NOT cause an update event on the ticket therefore you cannot then fire more triggers based upon this change.