You just need to add /multi to the end of the executable pathname, something like
"C:\username\Program Files\Ryzom\ryzom_client.exe" /multi
Don't use the shortcut to the .bat file though, it has to be the .exe file
"C:\username\Program Files\Ryzom\ryzom_client.exe" /multi
Don't use the shortcut to the .bat file though, it has to be the .exe file