1) Make sure the account has enough money for your withdrawal (same as EPS)
2) Activate overseas withdrawal function before departing from HK at any ATM machine in HK.
If it's Credit card with Union Pay Logo
1) You must know the maximum limits (for example, HK$100,xxx)
2) Password is 000000 if you have to enter password in the hospital or just sign the receipt. (this information is from UnionPay website)