to buy Octopus at Hung Hom Station, you must use HK$ cash instead of credit card. However, if you want to buy tourist Octopus (22/3 including airport express and three-day free subway), you can buy it with VISA card on the MTR website in advance and get a discount (note that the charge is HK$ or US$, depending on whether your card is HK$ or US$, RMB is not counted, because it is not a UnionPay channel). After the payment is successful, pick it up at the designated station.