I think it is better to use: Execute-MSI -Action Install -Path "$dirFiles\Jet.Setup.ExcelAddIn_x64.msi" -AddParameters "EXCELPLATFORM=x64 ACTIVATIONCODE=XXX-YYY-ZZZ"
.PARAMETER Parameters
Overrides the default parameters specified in the XML configuration file.
.PARAMETER AddParameters
Adds to the default parameters specified in the XML configuration file.