This depends on the setup between Boendeappen Avy and your property owner/manager.
In most cases, logging in is possible as soon as you have been registered in the property management system and the object/resident information has been sent to Boendeappen. Usually, this does not take more than a few days after you have signed your new contract and been registered in the property management system. This means you should be able to log in to the app before your move-in day.
Provided that Boendeappen's move-in service is activated, you will automatically receive an email and an SMS when you can log in, along with information on how to proceed.
If you have received a property code as the login method, you can log in as soon as you have received it.