It is not a ‘housing’ allowance as such, but a special allowance to cover any cost with living abroad. You can choose to use it all on accommodation or try to find a balance between accommodation and possible education fees for children or other expenses. Yo do not have to spend the entire allowance on accommodation. You can save the extra. Please note, it is not tax-deductible.
When you finally get a contract with a school, you will be given or you find all the forms to fill in.
The special allowance form will require proof that you are from abroad so you must keep some evidence should they wish to check. Mortgage , bank, utility bills, can be used as evidence as well as qualifications. It may also ask you questions about family members and where they live.
Basically you have to prove you are not a Hong Konger and you have no family ties here.
The special allowance will be paid with your salary each month. I can’t remember if you have to re-apply each year. You do not have to show proof of HK rent or other bills.
If you are not married, you both get to keep the allowance separately regardless of if you live together.