Jquery Multi-select ComboTree plugin: how to insert selected count label

Question

I'm using the Jquery Multi-select comboTree plugin and i would like to insert a selected elements count label inside combobox, but i'm not find a correct way to do this. I'm follow up the plugin documentation, that can be access here, but i can't developed a correct code to implement this.

Below, is the html code that receive data from plugin script:

<input type="text" id="select1" placeholder="Select">

and below is the plugin script that send data to html input mencioned above:

var myData = [
    {
      id: 0,
      title: 'value 1 ',
      subs: [
        {
          id: 10,
          title: 'subvalue 1'
        }, {
          id: 11,
          title: 'subvalue 2'
        }
      ]
    }, {
      id: 1,
      title: 'value 2',
      subs: [
        {
          id: 10,
          title: 'Item 2-1'
        }, {
          id: 11,
          title: 'Item 2-2'
        }, {
          id: 12,
          title: 'Item 2-3'
        }
      ]
    }, {
      id: 2,
      title: 'Item 3'
    },
];

var select1 = $('#select1').comboTree({

  source : myData,
  isMultiple: true

});

What i would like to do is insert a selected count label inside combo as image below:

How can i improve/implement my code to do this?