Yes, you can apply for pre-seating service at the point of ticketing and after the payment. After payment for ticket is completed, you can also purchase the service through Jin Air homepage 24 hours prior to departure.
However, if you have purchased your ticket through a travel agency, we politely advise you to click [My Trip] on our homepage and also call Jin Air Customer Center (82-1600-6200). (Some tickets purchased through specific travel agencies are not yet available so please contact the original channel of your ticket.)