[otrs] postmaster pre-filter: search in tickets

I wrote an postmaster pre-filter that searches in the tickets for an existing external ID, if it's existing a new article to this ticket is added. Otherwise a new ticket is added. My problem is that I have to define the Agent ID (UserID) in the Ticketsearch function in the pre-filter. How can I handle searches in all tickets and not only supportef by the selected agent? Thank you