Enmanuel Payano

Apologies if this is a late answer, but this method works for me:

client.get('ticketFields').then(function(data) {
      
      // Get all fields to query against
      var fields = data["ticketFields"];
      var CustomFieldName;

      // Iterate through each field
      for (var field in fields) {
          
          // Find the one that match
          if (fields[field].label == "Your Custom Field Name") {
             // console.log(fields[field].label);
             // console.log(fields[field].name);
             // Grab your custom field ID
             CustomFieldName = fields[field].name;
          }
      }

      // Do some custom field magic for your Zendesk
      client.set('ticket.customField:' + CustomFieldName, DataForYourField).then(function(data) {
          // https://SomeDomain.zendesk.com/api/v2/ticket_fields
          console.log(data); // { 'ticket.subject': 'Printer Overheating Incident' }
      });

});

Happy Coding!!