Written with people moving from Turkey in mind. The Portuguese rules still apply more broadly.
Easytoll: Tolls for Foreign Cars
If you drive a car with foreign plates into Portugal, you cannot use the Via Verde lanes immediately. You must register for "Easytoll" at the border welcome points (Welcome Centers).
You insert your credit card into the machine, and it links your license plate to the card for 30 days. Without this, you cannot pay for electronic tolls (SCUTs) legally, and fines will be sent to your home country.