Windows:
Download and install the EA App here. Run this program and follow its instructions to install EA App.
When the EA App is installed and you launch it for the first time, you will be prompted to create an EA account or log in to an existing account. If you do not have an EA account, select "Create Account" at the bottom of the EA App login window. Please note that all product keys you redeem will be tied to an account and cannot be used again, so it is important that you remember the login information for this account!
When you are logged into the EA App, select the "Library" tab on the left side. In there you will be able to select "Redeem code" on the right. Here you enter your product key.
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macOS:
Download and install Origin here. If you have a Windows PC, select "Download" under the Windows icon; Do you have a Mac, it's "Download" under the Apple icon. This will download a program to install Origin on your computer. Run this program and follow its instructions to install Origin.
When Origin is installed and you launch it for the first time, you will be prompted to create a Origin account, or log in to an existing Origin account. If you do not have a Origin account, select "Create an account" at the top of the Origin window. Please note that all product keys you redeem will be tied to an account and can NOT be used again, so remember to remember the login information for this account!
When you are logged in to Origin, select the "Origin" tab in the top bar. A menu will appear where you must select the item "redeem product code ...". Follow the instructions that appear and enter your product key when Origin asks for it. The product will now be added to your account and is ready to be downloaded.