Add a comment to the ticket

You must use an API call. client.invoke or client.set both deal with the agent interface data - assignee, requester, fields values, etc but to update a ticket you must do like this :

client.get('ticket').then(function(response) {
  let ticket = response['ticket'];
  console.log(ticket['id']);
  client.request({
    url: '/api/v2/tickets/' + ticket['id'],
    contentType: "application/json",
    type: 'PUT',
    data: JSON.stringify({
      "ticket": {
        "comment": {
          "body": "Your comment goes here",
          "public": false
        }
      }
    })
  }).then(function(response) {
    console.log(response)
  })
});