How to open Popup window from gridview edit button.

i am opening webform from button ,which is outside gridview,and Popup window is getting open,but i do not know that how to use same code in gridview on edit button ,and open window and pass pay_ID to that form which will open.

Below is code which i am using to open popup form 

<asp:Button ID="btn_New" runat="server"  Text="New" Width="74px" OnClick="btn_New_Click" />     <cc1:modalpopupextender ID="mp1" runat="server"  PopupControlID="Panl3" TargetControlID="btn_New" CancelControlID="btn_close" BackgroundCssClass="Background"> </cc1:modalpopupextender>                        <asp:Panel ID="Panl3" runat="server" CssClass="Popup" align="left" style = "display:none"><asp:Button ID="Button2" runat="server" Text="Close" /><iframe  style=" width: 980px; height: 750px;" id="irm1" src="partypy2.aspx" runat="server"></iframe><br/></asp:Panel>

Now i want to open same forms from gridview ,but with pass Pay_ID to webform,to retrieve data from database on webform,

<asp:GridView ID="gvpayment" runat="server"  BackColor="White" autogeneratecolumns="false"
                        BorderColor="#E7E7FF" BorderStyle="None" BorderWidth="1px" CellPadding="3" CssClass="myGridClass" 
                        Font-Names="Arial" GridLines="Horizontal" Height="198px"  style="margin-left: 3px" Width="807px" ShowFooter="True" OnRowDataBound="gvpayment_RowDataBound"
                        OnSelectedIndexChanged="gvpayment_SelectedIndexChanged" AutoGenerateSelectButton="True"><AlternatingRowStyle BackColor="#F7F7F7" /><Columns><%--      <asp:TemplateField HeaderText="Sr No" HeaderStyle-Width="5%" HeaderStyle-HorizontalAlign="Left"><ItemTemplate><%# Container.DataItemIndex + 1 %></ItemTemplate></asp:TemplateField>--%><asp:BoundField DataField="Pay_ID" ReadOnly="True" HeaderText="ID" /><%--          <asp:TemplateField HeaderText="ID"><ItemTemplate><asp:Label ID="Pay_ID" runat="server" Text='<%#Bind("Pay_ID")%>'></asp:Label></ItemTemplate></asp:TemplateField>--%><asp:TemplateField HeaderText="Party" Visible="true"><ItemTemplate><asp:Label ID="Level_Four_Name" runat="server" Text='<%#Bind("Level_Four_Name")%>'></asp:Label></ItemTemplate></asp:TemplateField><asp:TemplateField HeaderText="Remark" Visible="false"><ItemTemplate><asp:Label ID="Pay_Remarks" runat="server" Text='<%#Bind("Pay_Remarks")%>'></asp:Label></ItemTemplate></asp:TemplateField><asp:TemplateField HeaderText="Date"><ItemTemplate><asp:Label ID="Pay_Date" runat="server" Text='<%#Bind("Pay_Date")%>'></asp:Label></ItemTemplate></asp:TemplateField><asp:TemplateField HeaderText="Amount"><ItemTemplate><asp:Label ID="Pay_Amount" runat="server" Text='<%#Bind("Pay_Amount")%>'></asp:Label></ItemTemplate><FooterTemplate><asp:Label ID="lblTotalpay" runat="server"></asp:Label></FooterTemplate></asp:TemplateField><asp:TemplateField HeaderText="Edit"><ItemTemplate><asp:LinkButton ID="Pay_ID" Text='<%#Eval("Pay_ID") %>' runat="server" /></ItemTemplate></asp:TemplateField></Columns><FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" /><HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#F7F7F7" /><PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" /><RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" /><SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" /><SortedAscendingCellStyle BackColor="#F4F4FD" /><SortedAscendingHeaderStyle BackColor="#5A4C9D" /><SortedDescendingCellStyle BackColor="#D8D8F0" /><SortedDescendingHeaderStyle BackColor="#3E3277" /></asp:GridView>