The Activision ID is what allows you to cross-play with people on another console or PC. Without it, you have no way of adding players from PS4 if you are on Xbox or vice-versa.

Call of Duty account users have received messages that they now need to add their birthday and/or get parental consent to ...

Use your COD Activision ID details to log into the game. The accounts will get linked successfully. Once linked, players can keep track of their progression from WZ Mobile to MW3 and Warzone.

To find you Activision ID, you can open the Social page from the menu in Modern Warfare 2, Vanguard, Black Ops Cold War, Warzone, or Modern Warfare then view your profile ...

Once you’ve created your Activision ID, you need to use that to log in to Warzone 2.0. Using your Activision ID to invite friends can be done in both the PC and console versions of Warzone 2.0.