You can activate your Origin Games CD Key on Origin website online, or by downloading the Origin client for PC.
1, Origin Website
In case you use the online sign in: https://www.origin.com/, click on ‘My Game Library’, select ‘ADD A GAME’ option.
And click on ‘Redeem Product Code’.
2, Origin Client
Instead of ‘My Game Library’, you will need to select ‘Origin’ from the top Menu.
Then click on ‘Redeem Product Code’.
Now simply enter your game key in the ‘Enter Product Code’ field and click ‘Next’.