I do not condone pirating the game and recommend buying the game legit on sale or something, this is safer as the files are legit from steam not containing any viruses. If you however like me have the game on steam and wanted to track hours playing on private servers and wanted to know how to that is what this guide is for. If you wish to download plutonium I don't want to link it here because it may validate ToS however it's quite easier to find in a basic search. The other thing I want to note is this is not VAC bannable as what your doing is telling steam to load up a new .exe file when you click play on the game in steam. From here it opens plutonium which loads the game using it's own custom launcher based on the BO2 files you provide it and doesn't touch the BO2 steam client. I have been doing this for years now without issue, however of course do this at your own risk I am not responsible for anything.

Once downloaded you will have a .exe file called plutonium.exe next you need to navigate to

Windows :

C:\Program Files (x86)\Steam\steamapps\common/Call of Duty Black Ops II

Linux :

~/.local/share/steam/steamapps/common/Call of Duty Black Ops II

or

~/.steam/steam/steamapps/common/Call of Duty Black Ops II

or

~/snap/steam/common/.steam/steamapps/common/Call of Duty Black Ops II

if you have steam games on a different drive you will need to go to that directory instead of course.

Once here you want to find the .exe file called "t6mp.exe" and rename this to whatever to want I usually just name it "t6mp1.exe".

After this you want to copy the plutonium.exe file into this directory and rename it to "t6mp.exe" as that's what steam looks for when you press play on the game. After this you want to set the t6mp.exe (your old plutonium.exe file) to read only by right clicking it and going to properties and making it read only.

Steam likes to auto update games and will think this game needs an update sometimes read only works sometimes steam overrides it, on Linux you can make this permanent with the terminal command "sudo chattr +i t6mp.exe" if you open a terminal in this directory and to make it writable again just run the same command with -i instead of +i.

OPTIONAL STEP : Inside steam find Black Ops II Multiplayer and right click it or click the settings icon when the game is selected and find properties. Inside of properties go to the updates tab and you can change this to "only update this game when I launch it". This setting can help you as if it replaces the exe with the steam one again you will know and can change it back manually every time though. These are just the issues to deal with tracking hours on steam with 3rd party servers.

Now you can load the game through steam clicking play and it should load right into the plutonium launcher from here find the T6 Multiplayer section and it will show a blue button to setup. Click that and navigate back to the folder containing your BO2 files (where you just replaced the .exe file) select that folder and then it should be a green button that says Play and your all good to go. Steam will now track your hours through plutonium, Enjoy playing unmodded classic CoD games again :)