Seat reservations can be made without additional payment. To make a reservation, apply at any major station or Travel Service Center (Twinkle Plaza) or a reservation office (midori-no-madoguchi) at a JR HOKKAIDO station. There, present your PASS, and obtain your reserved-seat ticket before boarding.
A non-reserved seat can be used simply by presenting your JR Hokkaido Pass.
Seat reservations cannot be made before you receive the PASS in Japan.