This is a Installshield setup. So you can use the -r and -s switches.
setup.exe -r -f1\path\setup.iss -f2%TEMP%\ftppro.log
then to install silently use:
setup.exe -s -f1\path\setup.iss -f2%TEMP%\ftppro.log
I noticed that this installation needs the full path info to work, so if you use SMS like i do then send it out by assigning a drive letter and pointing the -f1 option to the drive, otherwise the setups keeps giving -5 and -3 errors.