Thank you for ordering your JR Pass with us!
Unfortunately it is only possible to make seat reservations once you arrive in Japan. However the end of January is not a very busy travel period and I am sure that you will be able to get seats, even if you just book them 5 minutes before departure. We also have a great blog post with more information regarding seat reservations.
It's a very nice itinerary btw! You are just in time for the Sapporo Snow Festival (or Yuki Matsuri in Japanse).