If you want to unlock the Burger King (BK) skin for free and double the XP bonus offered at Burger King, then you're have to follow the steps given below:
Order a Whopper Cheese menu as explained on the operation site
Your order placed and paid, you will receive a receipt
This receipt has a code at the bottom that you need to enter on the Call of Duty BK Redeem site (source). So don't throw away your receipt before you've redeemed the code.
Once the code is entered, you will be able to get the BK operator skin and also benefit from a double EXP bonus.