**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
commit=True,
            )
            flash("Marked {} on {} task instances".format(state, len(altered)))
            return redirect(origin)
        to_be_altered = set_state(
            tasks=[task],
            execution_date=execution_date,
            upstream=upstream,
            downstream=downstream,
            future=future,
            past=past,
            state=state,
            commit=False,
        )
        details = "\n".join([str(t) for t in to_be_altered])
        response = self.render_template(
            "airflow/confirm.html",
            message=f"Here's the list of task instances you are about to mark as {state}:",
            details=details,
        )
        return response
    @expose('/failed', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @action_logging
    def failed(self):