How to book Nohi bus from outside Japan

Understanding that Nobi bus can booking in advance. How to booking it from outside Japan and how to get it when arrive Japan? The route we would
book are:-

1. Tokyo-Takayama -Shirawago ( take night bus from Tokyo and morning bus from Takayama
2. Shirawago-Takayama (Leave from Shirawago around 11.00 am)

Please advise.

Unless you're travelling at a particularly peak time for Japanese travel, you shouldn't need to book before you get there. I took that route recently and bought the Matsu to Takayama ticket on the day; the Takayama to Kanzawa (via Shirakawa-go) ticket a couple of days before. On none of those trips were there no more than a handful of people on any bus.

I think you'd be incredibly unlucky to be unable to travel on the bus you want.

Be aware though that you can't but the tickets with credit cards - it's cash only. And I suggest that you have clearly written down which buses you want to catch when you go to the ticket office as the English language ability is variable.

They have a website in English.
But I'm not sure if it is helpful for you.

http://www.nouhibus.co.jp/english/index.html

Night time crusing from Shinjyuku Tokyo to Takayama is available in only summer season and the planned arrival time is very early morinig at Takayama.(around 5:30am)
It seems that you have to get another bus from Takayama to Shirakawago. But, the earilest available departure time for Shirakawago is 8:50am. It may not be benefitical for you and the first bus of a day may be overcrowded.
They also show their phone number of the reservation center on the website, however, I'm not sure whether or not they accept to get a phone call in English for reservation.