The half-price ticket benefit is available only to those who can provide the required documentation as per Brazilian laws, which does not extend the benefit to people who doesn't live in Brazil.
Foreigners should buy full-price tickets, and if you have a half-price ticket, you'll need to pay the difference. On the event day, simply go to the box office, show your ticket, and request the half-price supplement.
See more information by visiting this link.