**Description**
In the trigger page, there is a text area to enter (optional) conf json. It would be great if a sample JSON can be provided programatically while defining DAG

**Use case / motivation**

This will improve usability of the UI that triggers a DAG

tests/www/test_views.py
"""
        Test that the Rendered View is able to show rendered values
        even for TIs that have not yet executed
        """
        self.app.dag_bag = mock.MagicMock(
            **{'get_dag.return_value': SerializedDagModel.get(self.dag.dag_id).dag}
        )
        self.assertEqual(self.task2.bash_command, 'echo {{ fullname("Apache", "Airflow") | hello }}')
        url = 'rendered-templates?task_id=task2&dag_id=testdag&execution_date={}'.format(
            self.percent_encode(self.default_date)
        )
        resp = self.client.get(url, follow_redirects=True)
        self.check_content_not_in_response("echo Hello Apache Airflow", resp)
        self.check_content_in_response(
            "Webserver does not have access to User-defined Macros or Filters "
            "when Dag Serialization is enabled. Hence for the task that have not yet "
            "started running, please use &#39;airflow tasks render&#39; for debugging the "
            "rendering of template_fields.<br><br>OriginalError: no filter named &#39;hello&#39",
            resp,
        )
class TestTriggerDag(TestBase):
    def setUp(self):
        super().setUp()
        models.DagBag().get_dag("example_bash_operator").sync_to_db(session=self.session)
        self.session.commit()
    def test_trigger_dag_button_normal_exist(self):