I show how to boot directly into zombies using a steam shortcut even skipping the nagging parameter thing.
- Go to 'Add a Non-steam game'.
- Find 'steam.exe' (usually in "C:\Program Files (x86)\Steam\Steam.exe").
- Make sure its checkmarked then click 'Add selected programs'.
- Go to your Library tab in Steam.
- Find the new 'non-steam' game called 'Steam'.
- Right click it and then left click 'properties'.
- Change name to COD WW2 ZOMBIES for example.
- Change icon to whatever you want.
- Paste the following into "set launch options".
-applaunch 476620 +zombiesmode 1