Quote Originally Posted by waiming:
Has anyone succeeded in booking a BioNTech vaccine for a kid aged under 11 with no HKID, born overseas?
According to the government website you can just show up at the vaccination centre, explain your child's situation and they will be able to make a booking on the spot as a walk-in. It worked for my kids.

This works for other situations too where you can't make a booking. For example, my wife has an HKID but because her shots were all given overseas she couldn't make a booking on the website. She was able to do so as a walk-in after explaining her case.