How to add alert box when clicking on Approve or Reject button in Email

  const form = serverWidget.createForm({
                        title: ' ',
                    });
                    let alert = form.addField({
                        id: 'custpage_alert',
                        type: serverWidget.FieldType.INLINEHTML,
                        label: 'Alert'
                    });
alert.defaultValue = '<script>alert("The Purchase Invoice have been approved");window.close();</script>'