For the BSN registration appointment you need to bring proof of address (lease agreement, declaration of the owner of the address, or a purchase contract).
You cannot register at an address if the start date of your lease agreement/housing contract is after the date of your registration appointment. You cannot register using the address of a hotel or Air BnB.
If you have not yet found housing, the easiest solution is to temporarily use your employer's address. This is possible for 3 months maximum. When you find housing you will need to alert the municipality of your change of address.