Seat reservation before activating theJR pass

Hi, I am planning a holiday in May this year. I will be buying a JR Pass (whole of Japan). I like to reserve a seat on Fuji excursion train. Looking at the seat availability it seems to sold out long before the travel date. If I wait until I arrive in Japan then it’s too late to book for the next day. Various websites give conflicting answers regarding booking befor activating which can not be done from outside Japan.
What is best option available to book this journey
Regards