Go to https://redeem.meplay.com/user to login (if you don't have an account, please register first).
Select "Add Player".
Select "+Add new player ID for a game".
Enter UID FIFA Mobile, then select "Add Player".
UID can be found in the game in the settings menu.
After add UID and verified, select "Redeem Code" or can go to https://redeem.meplay.com/game/fifa.
Choose an UID that has previously been verified, enter the voucher code then select "REDEEM"