How to buy ticket in advance (online?) for travel between Guangzhou and Hong Kong?

When I try to use this site's search engine to find a train from Guangzhou to Hong Kong, none is found, even though I am aware that there is a train that runs from Guangzhou East Railway Station to Hung Hom Station in Kowloon. I would like to make advance reservations and purchase a ticket for such a train, but I cannot do it online if the site doesn't recognize the connection. How can I buy ticket in advance (online?) for travel between Guangzhou and Hong Kong? (And vice-versa?)