Fill your details in. You state on the form when you intend leaving Hong Kong.
Go to your local Home affairs dept office. Times and details below. Hand the form in. They will tell you to wait. Then they ask you to come in. You read the oath out in their presence and then they will ‘chop’ the form and sign it. You then submit that form with your form requesting the withdrawal of you MPF from whoever you have it with.
Home affairs offices and details of when and how you do it here. It’s quite easy and quick.
No idea...but I would guess you would need to be in Hong Kong to do your oath. I’d contact your MPF provider or search the Hong Kong Government web pages relating to this matter.