To redeem through the Origin client, first make sure you have downloaded and installed the Origin client. If you haven't, start by downloading the Origin client from www.origin.com/download.
Once you’ve created an Origin Account and signed in, just follow these steps:
1. Head to Origin in the menu bar on the top left of the client.
2. Select "Redeem Product Code"
3. Enter the Origin Key code provided to you
4. You will now be able to find your game in the Origin My Games library