First, and the easiest solution to show Activision that we're not the bunch of pirates that they think we are, you should have purchased and own the game on Steam. You can buy a copy of the game here.

On Steam Deck (now done by default) or on Steam Play (linux users) you can do this via the steam launch commands by changing the launch command to from within the Properties -> Launch options of your game:

On windows, much like the Steam Play version, you can set the following from within the Properties -> Launch options of your game. This should set the environment variable then immediately launch the game without setting it globally for your user. You may see a command window briefly while the game is launching.

On windows, it's a bit more complex since inlining an ENV variable isn't straightforward (and the launch command is handled separately). But, below are the instructions to get it up and running:


You can do this by hitting the windows button and typing 'environment`, then selecting the item for 'Edit environment variables for your account':




Alternatively you can access this window the long way by the following:

• Press Win + Pause
  
• Advanced System Settings on the right side (Windows 10)
  
• Environment Variables...

You should see a screen that looks like this:




Click 'New' under 'User variables' at the top

For the name use and the value
It should look like the following:



Click 'OK' and verify it's in the list at the top under 'User variables'


Close the steam client by clicking Steam -> Exit. Easy enough. Also, look at this clown with little faith.




That's it! You can launch Steam in offline mode and now bypass the unnecessary online check at the beginning of the game at launch. In some circumstances you may have to logout or restart your computer for the changes to take effect. This may or may not impact other games or Steam features so use at your own risk (I didn't notice any though).