JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://cdn.kendostatic.com/2013.2.918/js/kendo.all.min.js"></script>
<link rel="stylesheet" href="http://cdn.kendostatic.com/2013.2.918/styles/kendo.default.min.css">
<link rel="stylesheet" href="http://cdn.kendostatic.com/2013.2.918/styles/kendo.common.min.css">
Select ID's binding fine:<br />
<select data-role="multiselect" data-bind="source: selectData, value: selectedIDs" data-text-field="Name" data-value-field="ID"></select>

<br />Select ID's not binding to extended:<br />
<select data-role="multiselectcustom" data-bind="source: selectData, value: selectedIDs" data-text-field="Name" data-value-field="ID"></select>

<br /><br />
<small>*Strange but an integer, not array, binds fine to extended</small>

JavaScript

//EXTEND MULTISELECT WITH NOTHING MUCH
kendo.ui.plugin(kendo.ui.MultiSelect.extend({
    init: function (element, options) {
        kendo.ui.MultiSelect.fn.init.call(this, element, options);
    },
    options: {
        name: 'MultiSelectCustom'
    }
}));

kendo.data.binders.widget.multiselectcustom = kendo.data.binders.widget.multiselect;

var viewModel = kendo.observable({
    selectedIDs: [ 1, 3 ],
    selectData: [{
        Name: 'Bill Smith',
        ID: 1
    }, {
        Name: 'Jennifer Jones',
        ID: 2
    }, {
        Name: 'Tim Philips',
        ID: 3
    }]
});

kendo.bind('body', viewModel);