function activeren_form(field) 
{
  if (field.value.length==4 || field.value.length==5 || field.value.length==6)
  {
    document.forms['form_activeren'].action = 'http://www.mijnbladopdemat.nl/bladcadeau/activeren/stap1.vm';
  } else {
    document.forms['form_activeren'].action = 'http://www.mijnbladopdemat.nl/engine?app=bladopdemat&service=action:Aanmelden&cmd=login&succeeded=/bladcadeau/activeren/stap2.vm&failed=/bladcadeau/activeren/stap1.vm';
  }
}