How The HKMA uses US$ to buy HK$ changes the money supply?

HKMA issue Exchange Fund Bills (EFB) to capture (or to buy) HK$ in order to decrease the money supply of HK$ in the market. On the contrary, HKMA can redeem EFB to release (or to sell) HK$ in order to increase the money supply of HK$ in the market too.

You can see the issuance of EFB from the HKMA web site.