Citizens of EEA member states and Switzerland are entitled to work in Portugal without a permit, as long as their stay does not exceed 90 days.
UK citizens in principle are also work permits exempt for up to 90 days in a 180-day period. Please find additional information here: https://www.gov.uk/guidance/travel-to-portugal-for-work#short-term-visa-for-exempt-work
Most other foreigners (‘third-country nationals’) require authorisation to work in Portugal, and to apply for the corresponding visa. This should be organised by their employer, current or future.
Work permits for foreigners are managed by the provincial offices of the Ministry of Labour (‘Ministério do Trabalho, Solidariedade e Segurança Social (MTSS)’).
Decisions are usually issued within one month of receiving the application. If approved, the work permit is sent to the Portuguese embassy or consulate in the applicant’s country of residence, where they can then apply for their visa.
In addition, anyone working in Portugal must register with the Portuguese social security authorities ("Instituto da Segurança Social – ISS").