Buy ticket for Morioka to Hakodate

Hi everyone, I will be travelling in Japan around the Golden Week (end Apr-early May), arriving by air at Tokyo. Then I will go to Hakodate after a few days from Morioka. I want to reserve the seat for the train from Morioka - Hakodate as it is peak travelling period. However, i found the reservation website below only allow ticket pick up inside Hokkaido:
And people cannot use any kind of JR pass to reserve the seat.

I am ok to buy the ticket at normal price (not using JR pass) but the problem is i cannot reach Hokkaido before i get the ticket!! Anyone can have suggestion for me on what to do ? Thank you very much.

Erica