If you are just trying to determine the parameter so that you can use a startgame batch file...
Open the game using whatever launcher, then open a command line prompt. Enter “WMIC /OUTPUT:ProcessList.txt PROCESS get Caption,Commandline,Processid”. View command line options used to open each open program inside ProcessList.txt - specifically, look for the command line options used to open Game.exe. Then write your batch file using those command line options instead of startgame.