Now that the new housing law has been published, all existing AL owners have 2 months from tomorrow to confirm the AL is operational.
This will almost certainly be done from the page below, which will need to be changed for this new functionality. Foreign AL owners, if they want to do this themselves, will need a Chave Móvel Digital which, for foreigners, must be requested personally.
This can be done at the nearest Loja do Cidadão, by taking your ID (passport of national ID card), NIF and mobile phone.
https://eportugal.gov.pt/fichas-de-enquadramento/alojamento-local