You can create an association between Vak and Periode, doubleclick the association line and select it to be an n-m association.
One mind-change you have to make when starting with Mendix is that every association, on database-level, is in fact a separate table by itself. Seeing is believing, so run your app locally, then in Studio Pro click Console => ‘Start built-in database viewer’ and look for tables that have name that corresponds to one of your associations.