Custom field value change event

I am working on Zedesk server side App, There is a orderno custom filed in ticket form.
what i want to achieve is, get value of orderno  when its value got changed.

Previously, I have achieve something similar as following.

if (client) {
   client.on('ticket.requester.name.changed', function(e) {
      console.log("e === > ", e);
   });
}