Add Custom attribute for Message Start Event shape

Hello guys
I am working on bpmn-js-properties-panel 0.8.2 version.

I want to add extra properties for message start event so that the XML will be like

<bpmn2:startEvent id="StartEvent_1">
    <bpmn2:messageEventDefinition messageRef="office365EmailReciverTest" >
       <bpmn2:messgaeConfig messageId="123345" messageFolder="cd" ></bpmn2:messgaeConfig >

I have Defined the entry like

var entryFactory = require('bpmn-js-properties-panel/lib/factory/EntryFactory');

var is = require('bpmn-js/lib/util/ModelUtil').is;

module.exports = function(group, element) {
  // only return an entry, if the currently selected element is a start event
   if (is(element, 'bpmn:StartEvent') && element.eventDefinitionType === 'bpmn:MessageEventDefinition') {
      id : 'messageId',
      description : 'Message Id',
      label : 'messageId',
      modelProperty : 'messageId'

But it is adding an attribute for the start event tag.

What do I need to configure extra to get the formatted XML as I want?