def METHOD_NAME(self):
    # simplified from above just checking the kvk plugin
    step = FormStepFactory(
        form__slug="myform",
        form__authentication_backends=["demo-kvk"],
        form_definition__login_required=True,
    )
    form = step.form
    plugin = register["demo-kvk"]
    # demo plugins require staff users, see #1322
    user = StaffUserFactory.create()
    self.client.force_login(user=user)
    # we need an arbitrary request
    factory = RequestFactory()
    request = factory.get("/foo")
    url = plugin.get_start_url(request, form)
    next_url = quote("http://foo.bar")
    # start
    response = self.client.get(f"{url}?next={next_url}")
    self.assertRegex(response["content-type"], r"^text/html")
    self.assertEqual(response.status_code, 200)
    self.assertContains(response, "<form")
    # return
    url = plugin.get_return_url(request, form)
    self.assertNotIn(FORM_AUTH_SESSION_KEY, self.client.session)
    response = self.client.post(
        url, data={"next": "http://foo.bar", "kvk": "111222333"}
    )
    self.assertEqual(response.content, b"")
    self.assertEqual(response.status_code, 302)
    self.assertEqual(response["Location"], "http://foo.bar")
    self.assertIn(FORM_AUTH_SESSION_KEY, self.client.session)
    self.assertEqual("kvk", self.client.session[FORM_AUTH_SESSION_KEY]["attribute"])
    self.assertEqual(
        "111222333", self.client.session[FORM_AUTH_SESSION_KEY]["value"]
    )