having two buttons in aspx page. second button is in popup which is not working

<div class="detail_label">
                <asp:Button ID="btn_purchase" runat="server" Text="Add To Cart" OnClick="btn_purchase_Click" /></div>
          <div class="parentdiasble" runat="server">
            </div>
                <div id="popupdiv" runat="server" style="display: none" >

                            <table>
                                <tr>
                                    <td><span class="txt">Username: </span><span class="redstar">*</span></td>
                                    <td>
                                        <asp:TextBox ID="txtUserName" runat="server" placeholder=" UserName"
                                            Width="186px"></asp:TextBox>
                                        <asp:RequiredFieldValidator ID="rfvUserName" runat="server"
                                            ErrorMessage="Please enter  User Name" ControlToValidate="txtUserName" ValidationGroup="Login"
                                            Display="Dynamic" ForeColor="Red"></asp:RequiredFieldValidator>
                                    </td>
                                </tr>
                                <tr>
                                    <td><span class="txt">Password: <span class="redstar">*</span> &nbsp;&nbsp;</span></td>
                                    <td>
                                        <asp:TextBox ID="txtPwd" runat="server" TextMode="Password" placeholder=" Password" Width="186px"></asp:TextBox>
                                        <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
                                            ControlToValidate="txtPwd" Display="Dynamic" ValidationGroup="Login"
                                            ErrorMessage="Please enter Password" ForeColor="Red" SetFocusOnError="True"></asp:RequiredFieldValidator>
                                    </td>
                                </tr>
                                <tr>
                                    <td></td>
                                    <td>
                                        <asp:Button ID="btnSubmit" ValidationGroup="Login" runat="server" Text="Login" OnClick="btnSubmit_Click"/>
                                        <asp:Button ID="btnclose"  runat="server" Text="Cancel" CausesValidation="false" style="height: 26px" /></td>
                                </tr>

                                <tr>
                                    <td>&nbsp;</td>
                                    <td>&nbsp;</td>
                                </tr>

                            </table>
                      </div>

protected void btnSubmit_Click(object sender, EventArgs e)
    {
        SqlCommand cmd = new SqlCommand("popupid_sp", con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@name", txtUserName.Text);
        Class2.userid = Convert.ToInt32(cmd.ExecuteScalar());
        HttpCookie cookie = new HttpCookie("useridCookie");
        cookie["idCookie"] = Convert.ToString(Class2.userid);
        Response.Cookies.Add(cookie);
        Context.Server.Transfer("~/webpages/detail_page.aspx");
        
       
    }


  
    protected void btn_purchase_Click(object sender, EventArgs e)
    {
        if (Request.Cookies["idCookie"] == null)
        {
            ScriptManager.RegisterStartupScript(Page, typeof(Page), "Popup", "popuplogin()", true);
        }
        else
        {
            Context.Server.Transfer("~/webpages/detail_page.aspx");

        }
this is for javascript
function popuplogin()
       {

              $("#popupdiv").dialog({
               title: "login",
               width: 430,
               height: 250,
               modal: true,
               buttons: {
                   //close: function () {
                   //    __dopostback('btnsubmit', '');
                   //}
               }

           });

       }