To take advantage of the cross-progression benefits with Modern Warfare III and Warzone, make sure you log in to Warzone Mobile with the same account you use to play Modern Warfare III and Warzone.
Follow these steps to link a platform account to an Activision account:
Log in to your Activision account.
Hover over your account name at the top of the page and select LINKED ACCOUNTS.
Select Link Account next to the account you wish to link.
Please note: Activision accounts may only have one account linked per manufacturer.
Enter the Verification Code that will be sent to the email address associated with your Activision account.
Once again, select Link Account next to the account you wish to link.
You will be taken back to the PlayStation, Xbox, Steam, Blizzard, or Nintendo website to complete the account linking process.
Please note: Games with cross-progression store game progress on your Activision account, not your platform account. If you choose to link your platform account to another Activision account, your progress will not carry over. Your progress and content aren't lost. If you relink this platform account to the original Activision account, you can continue your progression and access your owned content.