New-ManagementRoleAssignment -Name "MailboxSearch" -Role "Mailbox Search" -User "..."
https://social.technet.microsoft.com/Forums/en-US/67b6dd5a-0a05-4136-a5ae-f09d5c35575b/newmailboxsearch-cmdlet-missing-parameters
New-MailboxSearch -SourceMailboxes Messler@contoso.com -TargetMailbox mailbox.suche@contoso.com -Senders someone@external.co.uk -LogLevel Full -Name "Messler"