Hi @Roger Roger ,
By my experiments, below are the Powershell commands and results you need to create contact:
New-MailContact -Name "Test User1" -ExternalEmailAddress "SMTP:test.user1@contoso.com" -Alias "test.user1.external" -DisplayName "Test User1" -FirstName "Test" -Lastname "user1" -OrganizationalUnit "OU=Contacts,DC=contoso,DC=com"
Reference: New-MailContact (ExchangePowerShell) | Microsoft Learn
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