Alternative way of joining by creating an Online Profile through CDKEY (credit: xSniP3Rr):
From xSniP3Rr:
Hey, I finally found a way to play online on Steam if you bought after 2021. First, you will need to have a working key. You can buy one online or you can buy the retail version and use the key that come with it. I bought one online and bought the retail version and both worked. You only have to make sure that the key you bought was
created before the problem. You don't have to worry if its the retail version cuz they are all working. When you have the working key you need to replace the steam one with the working one in the registry editor at HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Activision\Call of Duty WAW.
The key to change is "Codkey". You have to remove the hyphen otherwise the key will not work. The only problem is when you launch the game Steam will change the key every time. To fix that you need to remove permission of changing the key of all users. Right click on the Call of Duty WAW folder in the registry editor and click Permissions. After that you need to uncheck every permissions of all users except Read. Apply the settings and launch the game and you will be able create a profile if you did it correctly.