https://www.gamespot.com/articles/how-to-get-double-xp-in-call-of-duty-black-ops-4-b/1100-6462488/

Eating food and drinking liquid that is mostly bad for you and generally not very appealing to begin with could be your key to leveling up faster in Call of Duty: Black Ops 4. Because it is 2018 and This Is How Things Work Now, a number of retailers and restaurants have partnered with Activision for double XP promotions for the newest Call of Duty.

In the United States, retail juggernaut Walmart and others are selling Monster Energy drinks that come with codes for double XP. Buying the 4-pack gets you one hour of double XP, while those who buy the game and the energy drink pack get "MEGA XP," which gets you two hours of double XP. The Monster Energy drink campaign is also available in the UK.