You'd normally have to specify the path you want to open as well, unless Fluent adds that automatically.
This command will be a bit more efficient:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd Go NEW PATH="%1"
%1
is where the path should go, and may work as-is, if Fluent uses the same token style as File Explorer and the Windows registry.
Change NEW
TO NEWTAB
if you want a new tab instead of a new window.