In the inventory, under the panel on the right side that shows the items which your avatar is equipped with, there is a blue "Redeem Code" button. Pressing on the button causes a pop-up window to appear. In the middle of the window, there is an "Enter Item Code" field.

Cases of "shell shock" could be interpreted as either a physical or psychological injury. Although the United States' Department of Veterans Affairs still uses the term shell shock to describe certain aspects of PTSD, it is mostly a historical term, and is often considered to be the signature injury of the war.