There is no specific health authority requirement. Any EU-citizen or any citizen from another country can participate in trials taking place in Portugal without any authorization.
However, it would be very difficult for an investigator in Portugal to enroll a patient living in another country in a clinical trial, especially in a hospital from the National Health Service.
There are requirements from the Central Ethics Committee (CEIC):
The eHealth Digital Service Infrastructure (eHDSI) is an infrastructure ensuring the continuity of care for European citizens while they are travelling abroad in the EU. This gives EU countries the possibility to exchange health data in a secure, efficient and interoperable way.