Q:

ASP.NET Drop Down list , the button function not firing after the first click

ASP.Net Dropdownlist , on changing the value of drop down list and clicking the button it is not sending an upadted value to the New.aspx

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:DropDownList ID="ddl_test" runat="server" OnSelectedIndexChanged="ddl_test_SelectedIndexChanged" AutoPostBack="true">
        <asp:ListItem>First Item</asp:ListItem>
            <asp:ListItem>Second Item</asp:ListItem>
            <asp:ListItem>Third Item</asp:ListItem>

        </asp:DropDownList>
        <asp:HiddenField ID="hdf_ddl" runat="server" />
        <asp:Button ID="btn_test" runat="server" Text="Button" OnClick="Button1_Click" />
    </div>
    </form>
</body>
</html>

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class Test_DropDown : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string passValue = ddl_test.SelectedValue.ToString();
        string val = hdf_ddl.Value.ToString();
        btn_test.Attributes.Add("onclick", "window.open('New.aspx?ddlValue=" + val + "', 'OpenPopWindow','left=250,top=100,width=500,height=500,toolbar=1,resizable=0,status=0,menubar=0,scrollbars=1');return false;");
        //btn_test.OnClientClick = "window.open('New.aspx?ddlValue=" + val + "', 'OpenPopWindow','left=250,top=100,width=500,height=500,toolbar=1,resizable=0,status=0,menubar=0,scrollbars=1');return false;";

    }
    protected void ddl_test_SelectedIndexChanged(object sender, EventArgs e)
    {

        hdf_ddl.Value = ddl_test.SelectedValue.ToString();
    }
}

A:

Move the following code: 
btn_test.Attributes.Add("onclick", "window.open('New.aspx?ddlValue=" + val + "', 'OpenPopWindow','left=250,top=100,width=500,height=500,toolbar=1,resizable=0,status=0,menubar=0,scrollbars=1');return false;");

to your Page_Load event

