Select2 Demo

HTML

<link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/css/select2.min.css">
<script src="//cdnjs.cloudflare.com/ajax/libs/select2/4.0.0/js/select2.min.js"></script>
<select style="width: 300px;"></select>

JavaScript

var items = [
    {
        id: 'c1',
        text: 'Customer1',
        children: [
            { id: '1', text: 'Project1' },
            { id: '2', text: 'Project2' },
        ]
    },
    {
        id: 'c2',
        text: 'Customer2 ',
        children: [
            { id: '3', text: 'Project1' },
            { id: '4', text: 'Project2' },
        ]
    }
];

$('select').select2({
    placeholder: "Select customer/project",
    data: items
});