How do I redeem a key on origin?

Origin allows users to redeem a code for a game either via their Origin Client or via their online platform - See below for details

1. Redeem your code online
2. Log in with an EA Account.
3. Click My Game Library.
4. Click + Add a Game near the top of the screen.
5. Choose Redeem Product Code.
6. Type in the code.
7. Click Next.

For more information visit the EA help site -

https://help.ea.com/en-us/help/origin/origin/redeem-your-serial-code-in-origin/