In order to use Shift Codes, you must sign up for an account on the Borderlands VIP website, found here: https://borderlands.com/en-US/vip/
Once you create your account, you will gain access to the VIP program and also gain the ability to enter Shift Codes. Enter the desired code to receive the item in your game.

Effect	Password
Golden Key x1	ZRWBJ-ST6XR-CBFKT-JT3J3-FRXJ5
Golden Key x1	Z65B3-JCXX6-5JXW3-3B33J-9SWT6
Golden Key x1	ZFKJ3-TT6FF-KTFKT-T3JJT-JWX36
Golden Key x1	HXKBT-XJ6FR-WBRKJ-J3TTB-RSBHR
Golden Key x3	ZFKJ3-TT3BB-JTBJT-T3JJT-JWX9H
Golden Key x3	9XCBT-WBXFR-5TRWJ-JJJ33-TX53Z