Hi, I had a situation to modify the behaviour of many2many_tags widget. So I extended that widget to create a new widget n include my new field. You may refer my js code if its useful:

odoo.define('my_module.extension', function (require) {
"use strict";

var BasicModel = require('web.BasicModel');
var field_registry = require('web.field_registry');
var Notification = require('web.Notification');
var relational_fields = require('web.relational_fields');
var session = require('web.session');
var WebClient = require('web.WebClient');

var FieldMany2ManyTags = relational_fields.FieldMany2ManyTags;

var Many2ManyContactTags = FieldMany2ManyTags.extend({
      tag_template: "Many2ManyContactTags",

   fieldsToFetch: _.extend({}, FieldMany2ManyTags.prototype.fieldsToFetch, {
        //***fetching a new field****
        description: {type: 'text'}, 
    }),



});

field_registry.add('many2manycontacttags', Many2ManyContactTags);

});