At the event page:
http://www.raceroomers.pt/campeonatos/proracing-gtr3-2018-s01/

You must use REGISTER button to pay your registration.

After that go to SR Leagues: https://www.srleagues.com/login

SRLeagues is a championship manager where we organize our races. Register there and sign in to the event here: https://www.srleagues.com/champ/14/details