Refresh DropDownList on Button Click without page refresh using C# in ASP.Net

I want to refresh dropdownlist values without reload page

i add new values (importer Name), so i do not want to reload page again because avoid to refill fields on a page  

i am filling dropdown list on page load, then again i used indexchanged but it is not working properly 

below is code

        private void loadImporter()
        {
            con.Open();
            SqlDataAdapter adpbp = new SqlDataAdapter("select * from tbl_Importer  ", con);
            DataSet dsbp = new DataSet();
            adpbp.Fill(dsbp);
            ddlimporter.DataSource = dsbp.Tables[0];
            ddlimporter.DataTextField = "Im_Name";
            ddlimporter.DataValueField = "Im_ID";
            ddlimporter.DataBind();
            ddlimporter.Items.Insert(0, new ListItem("Select Importer ", "0"));
            con.Close();
        }
        protected void ddlimporter_SelectedIndexChanged(object sender, EventArgs e)
        {
            loadImporter();
        }

<asp:Content ID="Content1" ContentPlaceHolderID="MainContent" runat="server">
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" />
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>
    <script type="text/javascript">
        $(function () {
            $(".js-example-placeholder-single").select2({
                placeholder: "Select",
                allowClear: true
            });
        });
    </script>
    <br />
    <table class="nav-justified">
        <tr>
            <td style="width: 145px">&nbsp;</td>
            <td colspan="2">
                <h1 class="text-center" style="text-decoration: underline"><strong>Import Form</strong></h1>
            </td>
            <td>Date :               
                <asp:TextBox ID="txtentdate" runat="server"></asp:TextBox>
            </td>
        </tr>
        <tr>
            <td style="width: 145px">&nbsp;</td>
            <td style="width: 536px">&nbsp;</td>
            <td class="text-right">&nbsp;</td>
            <td>&nbsp;</td>
        </tr>
        <tr>
            <td style="width: 145px">BL No : </td>
            <td style="width: 536px">
                <asp:TextBox ID="txtblno" runat="server" class="form-control"> </asp:TextBox>

            </td>
            <td>BL Date :</td>
            <td>
                <asp:TextBox ID="txtbldate" runat="server" class="form-control"> </asp:TextBox>
            </td>
        </tr>
        <tr>
            <td style="width: 145px">&nbsp;</td>
            <td style="width: 536px">&nbsp;</td>
            <td>&nbsp;</td>
            <td>&nbsp;</td>
        </tr>
        <tr>
            <td style="width: 145px">IBN No :</td>
            <td style="width: 536px">
                <asp:TextBox ID="txtibnno" runat="server" class="form-control"> </asp:TextBox>

            </td>
            <td>IBN Date :</td>
            <td>
                <asp:TextBox ID="txtibndate" runat="server" class="form-control"> </asp:TextBox>
            </td>
        </tr>
        <tr>
            <td style="width: 145px">&nbsp;</td>
            <td style="width: 536px">&nbsp;</td>
            <td>&nbsp;</td>
            <td>&nbsp;</td>
        </tr>
        <tr>
            <td style="width: 145px">INDEX :</td>
            <td style="width: 536px">
                <asp:TextBox ID="txtindex" runat="server" class="form-control"> </asp:TextBox>
            </td>
            <td>Importer :</td>
            <td>
                <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                    <ContentTemplate>
                        <asp:DropDownList ID="ddlimporter" runat="server" AutoPostBack="true" CssClass="form-control js-example-placeholder-single" Height="85px" ToolTip="Select " Width="200px" OnSelectedIndexChanged="ddlimporter_SelectedIndexChanged">
                        </asp:DropDownList>
                    </ContentTemplate>
                    <Triggers>
                        <asp:AsyncPostBackTrigger ControlID="ddlimporter" EventName="SelectedIndexChanged" />
                    </Triggers>
                </asp:UpdatePanel>
                <asp:LinkButton ID="lbadd" runat="server" OnClick="lbadd_Click">Add</asp:LinkButton>
            </td>
        </tr>
    </table>
    <br />
</asp:Content>