It used to be apply through the agency, then it became apply through Norwegian & your course would be allocated one of the 3 agencies in the game.
To be honest, I have lost track of what is the "preferred" method now, but, whichever it is, it makes no difference, as you will be employed initially as a Contractor for 2 (sorry make that 3 ) years & then find yourself on a"local" contract at whichever base you are in . . . . allegedly.
But all & any of this could, & probably will, change, as it has done this last 2 years.