Parsing variable to add calendar rights

I try to add myself to all calendars with review rights in my company (Dutch language, hence the Agenda instead of Calendar).
Unfortunately $user:\Agenda  isn't parsed because the : will be added to the $user variable.

Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (Alias -ne 'Admin')} | ForEach-Object {
    $user = $_.SamAccountName
    Add-MailboxFolderPermission -identity $user:\Agenda -User me@company -AccessRights Reviewer
}

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How could I fix this?

Many thanks in advance!