$.widget("custom.leadsForConnections", {
    _create: function () {
        this.wrapper = $("<span>")
            .addClass("custom-combobox")
            .insertAfter(this.element);

        this.element.hide();
        this._createAutocomplete();
        this.element.bind("change", function () {
            this.input.val($(select).find("option:selected").text());
        });
    },
    _createAutocomplete: function () {
        var selected = this.element.children(":selected"),
            value = selected.val() ? selected.text() : "";
        this.input = $("<input>")
            .appendTo(this.wrapper)
            .val(value)
            .attr("title", "")
            .addClass("custom-combobox-input ui-widget ui-widget-content ui-state-default ui-corner-left")
            .autocomplete({
                delay: 600,
                minLength: 1,
                source: $.proxy(this, "_source")
            })
            .tooltip({
                classes: {
                    "ui-tooltip": "ui-state-highlight"
                }
            }).focus();

        this._on(this.input, {
            autocompleteselect: function (event, ui) {
                ui.item.option.selected = true;
                this._trigger("select", event, {
                    item: ui.item.option
                });
            },
        });
    },

    ...

});

<div class="g-hidden">
                        <div class="box-modal" id="add_connection_modal">
                            <div class="box-modal_close arcticmodal-close"><img id="closePopupImage"
                                                                                src="/images/icons/popup_icon_close.png"></div>
                            <div class="label">{'TEXT_NAME'|tr}:
                            </div>
                            <div class="ui-widget spn">
                                <select id="leadsForConnections" style="width:150px;">
                                    <option value="">Select one...</option>
                                </select>
                            </div>
                            <span  class="error_message" id="connection_error">Error</span>
                            <span  class="success_message" id="connection_success">Success</span>
                        </div>
                    </div>