This is going to be short. Hong Kong doesn't have postal or zip codes. Almost all the forms I've seen, including from huge companies like Amazon, flag this field as required. Most of the time simply entering HK works. Off the top of my head, I can't think of when HK didn't work, but I think it's happened once or twice.
Here's a good rule when validating a form containing a person's information: don't. Seriously, ask yourself how important the information truly is to what you are doing, and only require the minimum possible. Obviously a Postal Code can't be required if it doesn't exist and an invalid one works just as well.