PSScript-Copy First 3 Char of Attribute 2 Attribute

I need a Powershell script that will copy the first 3 characters of one AD attribute (Office Field / physicalDeliveryOfficeName) to another AD attribute (Custom Attribute13 / extensionattribute13) that can be run against all AD accounts in org (or all AD accounts that have office field populated)
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Following code should do it..
Import-Module Activedirectory
Get-ADuser -f * -Pr Office |?{$_.Office} |%{Set-ADUser $_.DistinguishedName -Add @{extensionAttribute13=$($_.Office.substring(0,3))}}

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itsystemsoperationsAuthor Commented:
I got an error when running that:

Set-ADUser : Multiple values were specified for an attribute that can have only one value:2 char:55

+ FullyQualifiedErrorId : Multiple values were specified for an attribute that can have only one value,Microsoft.ActiveDirectory.Management.Commands.SetADUser
Do you need to keep existing values for extensionAttribute13? if not, in code replace -Add with -Replace

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itsystemsoperationsAuthor Commented:
Does what I needed.   Thank you very much!
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