Direct Snapmap and Multiplayer Shortcuts in your Steam Library

In left bottom corner of steam interface choose "Add a game..." and there "Add a Non-Steam Game..."

Navigate to where Doom is installed, usually it's C:\Program Files (x86)\Steam\SteamApps\common or C:\Program Files\Steam\SteamApps\common on 32-bit machines.

You should now see Doom in your library once again.

Repeat the process once more to get two copies of the shortcut.

Change the name of the shortcut to whatever you want to recognise what it is. In my case I named it Doom Snapmap and Doom Multiplayer so that it would sort itself alphabetically closer to the original Doom Shortcut (which launches the Campaign).

Then add the launch option in the line "Target:"

This one will launch the game directly into the chosen gamemode.
+com_gamemode 3 for Snapmap
+com_gamemode 2 for Multiplayer

Optionally you can add launch options here, but they will be shared between the newly created shortcuts:

These are widely used:
+com_skipIntroVideo 1 +com_skipKeyPressOnLoadScreens 1 +menu_advanced_AllowAllSettings 1 +set m_smooth 0