Some items can be placed in your inventory by entering your redemption code through the in-game menu. To do this, follow the below steps:

Launch Rocket League.

Select Settings from the menu.

Select Extras.

Select Redeem Code.

Type in the in-game redemption code

All codes need to be entered exactly as they are printed. Make sure to include the dashes (-) and avoid adding extra spaces at the beginning or the end of the code

Select OK.