I've recently had the same task. This is my solution:
<input type="hidden" name="trBrand" id="trBrand" class="form-control"></input>
and js code
$('#trBrand').select2({
placeholder: 'choose something',
allowClear: true,
id: function(object) {
return object.text;
},
createSearchChoice:function(term, data) {
if ( $(data).filter( function() {
return this.text.localeCompare(term)===0;
}).length===0) {
return {id:term, text:term};
}
},
data:preload_data
});