Set Extented Attributes for users

Hello Everyone,

I'd like to get the list of user's mailboxes which are almost full and set ExtensionCustomAttribute5 to the value:

$Users = Get-MailboxDatabase | Get-MailboxStatistics | Where-Object {$_.StorageLimitStatus -match 'IssueWarning|ProhibitSend|MailboxDisabled'} | select Displayname

 ForEach($User in $Users)
  {
    $UserName = $User.Name
    Set-Mailbox -ID $_.users -ExtensionCustomAttribute5 shortcut
 }

Seems above is not working and giving me the error.

Where might be the problem?

Thanks!

KeJiaLi