The backtick version works but I realized my actual mistake is the missing double quotes near the end of ADDDEFAULT
Here is the corrected version with the missing " double quotes: Execute-MSI -Action "Install" -Path "$dirFiles\AEMinimal_13.0.3.1830.msi" -Parameters '/q INSTALLDIR="C:\Program Files (x86)\ARGUS Software" PSETUPTYPE=4 ADDDEFAULT="MainProduct,AEClient,feature.configutility,feature.exceladdin"'
I wanted to also point out that this backtick version works as well for those in the future that stumble upon this post trying to understand how to use the backticks: Execute-MSI -Action "Install" -Path "$dirFiles\AEMinimal_13.0.3.1830.msi" -Parameters "/q INSTALLDIR="C:\Program Files (x86)\ARGUS Software" PSETUPTYPE="4" ADDDEFAULT="MainProduct,AEClient,feature.configutility,feature.exceladdin""