Hi @Ozz ,
please try this:
$ou = "OU=Whatever,DC=MYDomain,DC=local"
Get-ADUser -Filter * -SearchBase $ou -Properties extensionattribute9 | Select-Object -ExpandProperty extensionattribute9
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(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten