It's not in the loot box or app. Log into your Tesla account in your browser. Click to your car then click Car Details, which shows all your options. You should see it show up there.
Mine says on the bottom:
"Free Supercharging until January 1, 2022"
Mine also lists "Full Self-Driving Capability" on top of that due to a 3 month trial (I took delivery on 12/31/2020).