Running 7-Zip from within a Powershell script

Simply prefix the command with an ampersand

& "C:\Program Files\7-Zip\7z.exe" -mx=9 a "c:\BackupFolder\backup.zip" "c:\BackupFrom\backMeUp.txt"

Maybe a simpler solution is to run 7-zip on your Powershell via cmd:

cmd /c 7za ...

Found this script and adapted it to your needs. Can you please try:

$7zipPath = "$env:ProgramFiles\7-Zip\7z.exe"

if (-not (Test-Path -Path $7zipPath -PathType Leaf)) {
    throw "7 zip file '$7zipPath' not found"
}

Set-Alias 7zip $7zipPath

$Source = "c:\BackupFrom\backMeUp.txt"
$Target = "c:\BackupFolder\backup.zip"

7zip a -mx=9 $Target $Source

put "&" special character before 7z command. Example: &7z ...