http://store.steampowered.com/app/332480/

1. Register on http://www.splitplay.com/

2. Login.

3. Go to http://www.splitplay.com/pt/games/phoenix-force#

4. Press the green button.

5. Get the key ( Sometimes the keys will be replenished, so you've to wait a while. )

(Proof for more keys: https://twitter.com/splitplay )

Have fun