you can apply in HK too

if you are not subject to security check, it will approved on the spot. However, it needs to be mailed out. So you can arrange it to send to HK general postal office for pickup. So, you can get it on the third day or at lastest the fourth day. If you never been to hk before, this may be a good choice as you can do a couple of days of sightseeing when you wait.