I haven't seen it hit my account. Picked up 12/23. Hmm.
It will not show up anywhere in your account i.e. either Lootbox or your Tesla Online account. The only way to confirm is go to a Supercharger and use it, it should not show up in your account/credit card charge. Also on the charging screen in the car it will not show any amount for the session. I hope this helps to clarify how the free supercharging will work