how to reserve box seat train

how to reserve box seat in resort shirakami train.

It's in the second car. But cannot book in the JR EAST website.

Or. went to reserve at the JR office. when i arrive.

I couldn't find JR East website about the booking of Resort Shirakami box seat, but this site

https://www.toretabi.jp/railway_info/shirakami-reservation.html

mentions that it can be only done in the "Midori no Madoguchi", JR's ticket office in the station.