Remote Desktop Manager simply calls the command line interface (CLI) with supported parameters. At this time (v4.9), the executable can be found in
C:\Program Files\Dell SonicWALL\Global VPN Client\SWGVC.exe
Simply call it as follows
SWGVC.exe /E connection_name /U username /P password
The CLI must be able to connect, Remote Desktop Manager doesn't perform anything additional to make it work.