On the job page, before pickup, press Give to subcontractor. Choose the subcontractor and what they are paid; their usual share of the price is suggested. A carrier not set up yet can be added on the spot with New subcontractor next to the list (Admins, Accounts, and anyone an Admin gave "Add and edit subcontractors" on the Staff page); it is chosen once saved. The job moves to "Subcontracted" on the dispatch board.
Record their progress with Mark picked up and Mark delivered (or a failed attempt), since subcontractors do not use the app. Take back returns the job to your own drivers.