Gets the associated launch parameter if the game is run via steam://run/appid/?param1=value1;param2=value2;param3=value3 etc. Parameter names starting with the character '@' are reserved for internal use and will always return an empty string. Parameter names starting with an underscore '_' are reserved for steam features -- they can be queried by the game, but it is advised that you not param names beginning with an underscore for your own features.