@using (Ajax.BeginForm("AddOrderItem", "Order", new AjaxOptions { UpdateTargetId = "tabItems", HttpMethod = "Post" }))
{
    //здесь поля для ввода
}
<script type="text/javascript">
    $(".select2").each(function () {
        var url = this.dataset.url;
        $(this).select2({
            allowClear: true,
            minimumInputLength: 1,  // minimumInputLength for sending ajax request to server
            width: 'resolve',   // to adjust proper width of select2 wrapped elements
            ajax: {
                url: url,
                type: "GET",
                dataType: 'json',
                data: function (term) {
                    return {
                        searchStr: term,
                    };
                },
                results: function (data) {
                    return { results: data.list }; // data.CountryList returning json data from Controlle
                }
            }
        })
    })
</script>