location = forms.ModelChoiceField(queryset=Location.objects.all())

def __init__(self, *args, **kwargs):
    super(MyCustomUserCreationForm, self).__init__(*args, **kwargs)
rel = ManyToOneRel(self.instance.location.model, 'id')
self.fields['location'].widget = RelatedFieldWidgetWrapper(self.fields[
    'location'].widget, rel, self.admin_site)
Django ModelChoiceField has no plus button