How to do AsyncPostBackTrigger for the LinkButton in the Repeater

In my page, I have an LinkButton inside repeater, but the UpdatePanel cannot find the LinkButton to AsyncPostBackTrigger.

Here is mycode.aspx

<asp:ScriptManager ID="Test1" runat="server" />
<asp:UpdatePanel ID="TestUpdate" runat="server" UpdateMode="Always">
<ContentTemplate>
<table width="100%">
<tr valign="top">
    <td width="50%">
        <asp:Repeater ID="productList" runat="server" onitemcommand="productList_ItemCommand">
        <HeaderTemplate>
        <ul type="disc">
        </HeaderTemplate>
        <ItemTemplate>
        <li>
            <asp:Label id="L1" runat="server" Text='<%# Eval("productName") %>'></asp:Label><br />
            Price:
            <asp:Label runat="server" Text='<%# Eval("productPrice") %>' ></asp:Label>&nbsp;Bath<br />
            <img alt="" src="Images/product/product<%# Eval("productID") %>.png" style="width: 200px; height: 130px" /><br />
            <asp:TextBox ID="num_product" runat="server" Text="0"></asp:TextBox><br />
            <asp:LinkButton ID="order_button" runat="server"><img alt="" src="~/Images/button/order.png" /></asp:LinkButton>
        </li>
        </ItemTemplate>
        <FooterTemplate>
        </ul>
        </FooterTemplate>
        </asp:Repeater> 
    <td>
    <span class="labelText">Order list</span>
        <asp:BulletedList ID="orderList" runat="server" BulletStyle="Numbered">
        </asp:BulletedList> 
    </td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>

Here is mycode.aspx.cs

protected void productList_ItemCommand(object source, RepeaterCommandEventArgs e)
    {
        //button
        /*LinkButton btn = new LinkButton();
        btn.ID = "order_button";