Either or. You can add priming sugar (not sure on the exact amount, but it's considerably less than you would do when bottling), seal up and purge the air from the keg and allow it to carb and condition in the keg. Or you can do like most folks do and simply transfer the beer to the keg, put the keg in the fridge under serving pressure, and wait 10-14 days, and have carbed beer. There are a couple other shortcut methods, but they can lead to overcarbed beer - the set and forget method of force-carbing is the safest and most reliable.