If the code is an origin code, go to the origin tab in your origin account and click "redeem code."
Also, right now there is free gametime for Sims 4 right now. To get that you go to the store tab of origin, and click on the free games tab, then click on "gametime." From there you can download the free game too. Note: the game time trial is for 48 hours only, so once you start playing, you will have 48 hours from when you start playing.
Hope this helps!
