Hi there!
Steam cd keys are not more longer active to multiplayer since endind 2020. This is because they got out of MP CdKeys in Activision.
Some time ago, they added some more cdkeys for a short time, like 2021 October until 2022 December.
Since then the ONLY way to activate a Steam CDKEY for MP is an old CDKEY that WAS ACTIVE BEFORE 2020 or an ACCOUNT STEAM ACTIVE BEFORE 2020.
But...... The Thing and great news if you still want play MP is you can bought a CDKEY from retail version disk, no matter time was made, and get the CDKEY to activate it.
Then you can add that game, because you will have to install it as cd install again (new install) as usual, then add the route to the .exe to your steam games library and you will have the game in your library
steam to start it from there, or desktop icon for launch game.
Also, there is a trick for the register windows, where you can change the cdkwy actually set up in your game and then copy the steam account game register adress and paste there the new cdKey wich will
allow you to play with the original Steam game you had installed before.
THe route for Windows Registry to change your ID and CDKEY is:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Activision\Call of Duty WAW
Note: On 32-bit systems, it may be HKEY_LOCAL_MACHINE\SOFTWARE\Activision\Call of Duty WAW.
Here a way to go guide to help:
To change the CD Key for Call of Duty: World at War (CoD WaW) via the Windows Registry—often required to fix "key in use" or "invalid key" errors when playing online—you must modify the registry entry and restrict permissions to prevent Steam from reverting it.
Registry Key Location
The CD key is located in the Windows Registry at:
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Activision\Call of Duty WAW
Note: On 32-bit systems, it may be HKEY_LOCAL_MACHINE\SOFTWARE\Activision\Call of Duty WAW.
Steps to Change CD Key
Open Registry Editor: Press Windows Key + R, type regedit, and hit Enter.
Navigate: Go to HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Activision\Call of Duty WAW.
Modify Codkey: Find the key named codkey on the right side. Right-click it, select Modify, and paste your new working key.
Important: Enter the key without hyphens (e.g., ABCD1234ABCD1234ABCD).
Prevent Reversion (Permissions): Steam often resets this key. To prevent this, right-click the Call of Duty WAW folder on the left pane and select Permissions.
Restrict Access: Click Advanced. Uncheck "Allow" for all users (or just for Steam/SYSTEM) and ensure only Read is permitted. Click Apply.
Fixing "Profile Could Not Be Created"
If the key change does not allow you to create an online profile, it may be due to improper key formatting or permission issues.
Remove Hyphens: Ensure the codkey in the registry has no hyphens (-).
Run as Admin: Run Registry Editor as an Administrator to ensure permission changes apply.
Also as has been said before check this guide for Steam users
https://steamcommunity.com/sharedfiles/filedetails/?id=2910762686
Greetings
Mikel