name in passport

Hello Travel China Guide.. I am a Malaysian foreign, will be travelling by HSR from Yangshuo to Shenzhen.. My name is Olivia Toh Li Nah as per Passport. My Surname is "Toh" (2nd word in the Passport). In the sample ID indicates the Surname "Ong" is the 1st word. How should I fill in my name when i purchase the HSR tickets? Appreciate your reply soonest. TQ