Beppu Kitahama to Fukuoka Airport

How can I make reservations or purchase the bus ticket from Beppu Kitahama to Fukuoka airport prior to my arrival in Beppu? Can I purchase it at Fukuoka airport? The Nishitetsu bus website doesn't seem to have any online reservation.

Yes, you can purchase at Fukuoka Airport. Online booking is in Japanese only, though phone booking is probably possible in English.

It looks you could use this online reservation site in English.
https://global.atbus-de.com/route_lists/?locale=en

The official site says the part between Beppu Kitahama and Fukuoka Airport is covered by TOYONOKUNI-GO (nonstop). A reservation is required and they start to accept it one month before. You can use the WEB (the above) or toll-free telephone (0120-489-939).
http://www.nishitetsu.jp/bus/highwaybus/rosen/toyokuni.html

I may ask the hotel to arrange the booking or just follow the official guide in English:
http://www.nishitetsu.jp/en/highway_bus/

Luisjp,thank you very much for the English website on the highway bus reservation! It is most helpful as I can purchase my ticket in advance .Much appreciated.
Evelyn