How to use "Listener" properties in BPMN

[rbajaniya]

I have add Execution Listener and generate XML.

<bpmn:task id="Task1">
    <bpmn:extensionElements>
      <camunda:executionListener event="start">
        <camunda:script scriptFormat="javascript">function(){alert(1)}</camunda:script>
      </camunda:executionListener>
    </bpmn:extensionElements>
   <bpmn:incoming>SequenceFlow_1hvxda3</bpmn:incoming>
</bpmn:task>

How to use "camunda:executionListener" properties?