Without a specific code or more details, here are some general tips:

For pure XP, mission objectives matter more than kills. Dubai is the fastest map. Pick the "Payout Objective" that gives the most XP (e.g., "Silent Assassin - No Firearms") and rush the penthouse guards. You can finish a Dubai mission in 90 seconds for 1,000+ XP.

Don't ignore your safehouse. While not a "cheat," there is an exclusive XP loop:

Many players don't know this: You can craft a Concussion Device in the safehouse basement using junk items found in missions. Concussion devices are required for several high-XP prestige objectives. This turns a "weakness" into a rapid leveling tool.