HTTP Task posting to api, what ID to use for callback

I’m creating a flow where an external Inquiry app is used,
I have created the flow to POST a json to the app,
the app needs to have some ID to POST the answer at a later stage,

preferable I would include the ID or URL in the initial post, but I can’t find the actual variable I should include.

I start the flow with a http POST to flowable, and IT returns the id’s and url, but I just can’t figure out how to access those in the “http task” …

{
  'email': ${vervoerderemail}, 
  'reference': ${mitozreference},
  'pickupdate': ${pickupdate},
  'pickuptime': ${pickuptime},
  'info': ${shipmentinfo}
}