Does PoA should be registerd by a notary in Belgium?

Pay a notary and get a legitimate answer.

If you want your tenant to pay the rent to someone else, just write that in your rent contract. You do not need a Power of Attorney.

if you want to authorise your friend then give him a PROCURATION. Normally you add a copy of your ID on top and then write 'Procuration' and then the text like Je, le soussigné, authorise par la présente M. Name Surname à recevoir le loyer pour le bien sis à Address of the place, émettre les reçus, signer à mon nom. Fait à Bruxelles le 29/04/2020 and Signature. It is a document 'sous seing privé' and does not require a notary.
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