**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
with conf_vars({('logging', 'logging_config_class'): 'airflow_local_settings.LOGGING_CONFIG'}):
            self.app = application.create_app(testing=True)
            self.appbuilder = self.app.appbuilder  
            self.app.config['WTF_CSRF_ENABLED'] = False
            self.client = self.app.test_client()
            settings.configure_orm()
            self.login()
            dagbag = self.app.dag_bag
            dag = DAG(self.DAG_ID, start_date=self.DEFAULT_DATE)
            dag_removed = DAG(self.DAG_ID_REMOVED, start_date=self.DEFAULT_DATE)
            dagbag.bag_dag(dag=dag, root_dag=dag)
            dagbag.bag_dag(dag=dag_removed, root_dag=dag_removed)
            
            with mock.patch.object(settings, "STORE_DAG_CODE", False):
                dag.sync_to_db()
                dag_removed.sync_to_db()
                dagbag.sync_to_db()
            with create_session() as session:
                self.ti = TaskInstance(
                    task=DummyOperator(task_id=self.TASK_ID, dag=dag), execution_date=self.DEFAULT_DATE
                )
                self.ti.try_number = 1
                self.ti_removed_dag = TaskInstance(
                    task=DummyOperator(task_id=self.TASK_ID, dag=dag_removed),
                    execution_date=self.DEFAULT_DATE,
                )
                self.ti_removed_dag.try_number = 1
                session.merge(self.ti)
                session.merge(self.ti_removed_dag)
    def tearDown(self):