If you wish to become a freelancer (Independent Entrepreneur) in Portugal being foreigners, you should be in Portugal on a valid legal visa, or maybe you exempt from permit. Is eligible to become a freelancer after self-declaration(register) with Portugusees taxation & finance department.

Documents:

obtained a legal entry deceleration from SEF ( Portuguese immigration department)

NIF(Número de Identificação Fiscal or Número de Contribuinte) form Portuguese tax authorities,

Hold documents of the resident certificate (Junta) from local municipal corporation so-called (junta de freguesia)

Having a personal bank account.

While opening self-employment activities in Portugal:
While beginning the operation, you should mention here the service of your business/work you are going to start, and a code of this activity shall be mentioned in your registration document.
download pdf

source