If I understand you correctly, you don't need a popup, you just trying to activate some div on your page. You will need to make this div server side

<div class="popup" runat="server" style="display:none" id="divPop">
 <asp:TextBox ID="_lname" runat="server"></asp:TextBox>

    <asp:Button ID="Button1" 
        runat="server" 
        Text="save" OnClick="Button1_Click" 
         />
</div>

and then in your server side click

protected void btnOpenPopupWindow_Click(object sender, EventArgs e)
{
  divPop.Attributes.Add("style", "display:block);
}

this will make your div visible

EDIT better solution in your case would be using asp panel instead of div.

<asp:panel id="divPop" visible="false" runat="server">
    <asp:TextBox ID="_lname" runat="server"></asp:TextBox>

      <asp:Button ID="Button1" 
    runat="server" 
    Text="save" OnClick="Button1_Click" 
     />
</asp:panel>

and then you will be able to manipulate visibility in your server side code very easy

protected void btnOpenPopupWindow_Click(object sender, EventArgs e)
{
  divPop.Visible=true;
}

In jQuery, you can open a popup by something like this, here you can pass your html in .html:

    var customDialog = function (options) {
        $('<div></div>').appendTo('body')
                        .html('<input type="checkbox" id="myCheckBox" />Test Checkbox<div style="margin-top: 15px; font-weight: bold;">' + options.message + '</div>')
                        .dialog({
                            modal: true,
                            title: options.title || 'Custom Popup Box', zIndex: 10000, autoOpen: true,
                            width: 'auto', resizable: false,
                            buttons: {
                                Ok: function () {
                                    $(this).dialog("close");
                                },
                            },
                            close: function (event, ui) {
                                $(this).remove();
                            }
                        });
};

$('#myButton').click(function() { customDialog({message: 'your input message '}); });

Now you can call this method, on button click etc

customDialog({message: 'Test Message'});

Check this JS Fiddle [EDIT] To call a jquery function from anchor tag check this fiddle

you can use jquery ui dialogs for this. Check out this sample

You can use ClientScript.RegisterStartupScript

In C#

protected void Button1_Click(object sender, EventArgs e)
{
    string queryString = "test.aspx" ;
    string newWin ="window.open('" + queryString + "');";
    ClientScript.RegisterStartupScript(this.GetType(), "pop", newWin, true);
}

In VB

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)

        Dim queryString As String = "test.aspx" 
        Dim newWin As String = "window.open('" & queryString & "');"
        ClientScript.RegisterStartupScript(Me.GetType(), "pop", newWin, True)

End Sub

Use this code

 <asp:TableCell>
       <asp:Button ID="btnSearch" Text="Search" OnClientClick="openwd();" runat="server"/>
  </asp:TableCell>


 <script type = "text/javascript>
   function openwd() {
   window.open("OnPopUp.aspx");
   return false;
   }
  </script>