At all of the metro station of HK, you could purchase a Hong Kong Octopus Card(https://www.travelchinaguide.com/cityguides/hongkong/transportation/octopus-card.htm) and put it in use immediately. You don't have to show any travel documents or applicication since the card could be purchased freely. Just pay the staff HKD50 for deposit and add some value to your card as you wish. When you leave from Hong Kong, you could withdraw the deposite and the balance in your card.