**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

airflow/www/views.py
"""Clears the action."""
        try:
            dag_to_tis = {}
            for ti in task_instances:
                dag = current_app.dag_bag.get_dag(ti.dag_id)
                task_instances_to_clean = dag_to_tis.setdefault(dag, [])
                task_instances_to_clean.append(ti)
            for dag, task_instances_list in dag_to_tis.items():
                models.clear_task_instances(task_instances_list, session, dag=dag)
            session.commit()
            flash("{} task instances have been cleared".format(len(task_instances)))
            self.update_redirect()
            return redirect(self.get_redirect())
        except Exception:  
            flash('Failed to clear task instances', 'error')
    @provide_session
    def set_task_instance_state(self, tis, target_state, session=None):