You can put in a script file and call the script.
.\myscript.ps1
You canput it in a variable like this:
$sb=[scriptblock]::Create('Execute-MSI -Action Install -Path $Var_SourceFiles\$Itemvar_Setup_Parameter[0] -transform $Itemvar_Setup_Parameter[1] -Parameters $Itemvar_Setup_Parameter[2] -LogName $var_PackageLongName')
$sb.Invoke()
You cannot pu tit in a variable and run it except.
Your command also leaves a lot to be desired in a PowerShell session.
¯\_(ツ)_/¯