While there is no official way of starting Steam in Offline mode w/o showing the pop-up window you mention in your question, you might observe that when you switch Steam client from Online to Offline mode, you get no such pop-up.
The reason why this happens is because Steam changes one registry entry that suppress displaying of that window. You can check it by yourself by downloading program called Process Monitor from Microsoft website and setting it to monitor registry operations of steam.exe while you switch to offline mode.
Only problem is that this registry value (
HKCU\Software\Valve\Steam) is reset to default when you exit the steam, resulting in displaying this "Please go online" dialog next time you start Steam application.
I came up with small batch file as a workaround for problems mentioned above. It's simple two code line file that first changes value of registry entry responsible for skipping offline dialog and then starts steam client:
@reg add HKCU\Software\Valve\Steam /v SkipStayOfflineDialog /t REG_DWORD /d 0 /f >nul
Just save this as a SteamOffline.bat (name of file is unimportant, only extension is) and place it in your Steam directory.
You can then use it for launching Steam.
You could also make a shortcut for this .bat and add custom icon for it, so you could pin it to Start Menu or place it on desktop. You could also set shortcut to run minimized if you don't want a cmd.exe window popping up in the middle of the screen:
- In would be a good idea to go online once in a while to fetch new Steam and game updates;
- You are modifying a registry with this file, if that statement scares you, then please do not use this method :)
- You can add -silent after steam.exe in bat file to hide the "Connecting Steam account: Username" window;
- You can always start Steam normally by executing Steam.exe or it's default shortcut.