bahnjee is right, with the second example not working. In PowerShell the single quote implies an absolute value, whereas a double quote will evaluate variable within it. Even double quotes in single quotes are considered absolute values.
Building on bahnjee first example, you could also use double quotes to achieve a similar result:
Execute-Process -Path "$dirFiles\InfoAsset+Planner+Setup+2020.1.exe" -Parameters "/s /f1""$dirSupportFiles\setup.iss"" /f2""$appname-Install.log"""
You may also want to specify the folder where the log file is created (see the examples below).
I keep a crib sheet with installation commands for common setup packages to run silently and create log files. Some of these are in the documentation. They give me a starting point when I package new software. Hopefully they may be of use:
## *** Examples of an EXE install *** ##
Execute-Process -Path "MySetup.exe" -Parameters '/S'
Execute-Process -Path '<application>.exe' -Parameters '/quiet' -WaitForMsiExec:$true
# Launch InstallShield "MySetup.exe" with embedded MSI and force log files to the logging folder.
Execute-Process -Path 'MySetup.exe' -Parameters "/s /v`"ALLUSERS=1 /qn /L*v \`"$configToolkitLogDir\$installName.log\`"`""
# Launch InstallShield "MySetup.exe" and force log files to the logging folder.
Execute-Process -Path 'MySetup.exe' -Parameters "-s -f2`"$configToolkitLogDir\$installName.log`""
# Launch InnoSetup silently
Execute-Process -Path "MySetup.exe" -Parameters "/SP- /VERYSILENT /SUPPRESSMSGBOXES /NORESTART /LOG=`"$configToolkitLogDir\$installName.log`""