If the two tables are linked in a relationship:
VoorCal::project_id = Werkbon::Project_id
If [Not IsEmpty ( Werkbon::Project_id ) ]
Go To Related Record [Show only related records; From table: Werkbon; Using layout: "Werkbon" (Werkbon) ]
Show Omitted Only
Go To Layout [ "Werkbon" (Werkbon)]
Show All Records
If executed from a layout based on VoorCal, will pull up all records in Werkbon that do not have a matching project_id.