How to get value in a textbox inside modalpopup extender which is inside a gridview

Hi,

I have a gridview with an approve button.when i click on approve button a modal popup will open.inthe modal popup i will add some commnets and i will click on another button.

I need to get the comments added when i click on last button

  <asp:GridView ID="grd_ApproveTable" runat="server"  Width="100%"
            AutoGenerateColumns="false" BackColor="White" BorderColor="#333333"
            BorderStyle="Outset" BorderWidth="2px" ShowHeaderWhenEmpty="True"  >
        <Columns>
        <asp:BoundField HeaderText=" Registration ID " DataField="ReservationID" HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White"/>
         <asp:BoundField HeaderText=" Requestor Name " DataField="Username" HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White"/>
         <asp:BoundField HeaderText=" Email " DataField="Email" HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White" />
         <asp:BoundField HeaderText=" Project Name " DataField="ProjectName" HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White" />
      <asp:TemplateField HeaderText=" Approve " HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White">
      <ItemTemplate>
        
          <asp:ImageButton ID="btn_Approved" runat="server" ImageUrl="images/Approve.png" Height ="25px"     />
           <asp:Panel ID="pnl_Comments" runat="server">
        <asp:TextBox ID="txt_CommentsApproved" runat="server" TextMode="MultiLine" Height="150px" Width="300px"></asp:TextBox>
        <br />
         <asp:ImageButton ID="btn_Approve" runat="server" ImageUrl="images/Approve.png" Height ="25px"  OnClick="btn_Approve_Click" CommandName="Approve" CommandArgument='<%# Eval("ReservationID") %>'    />
        </asp:Panel>
         <ajaxToolkit:ModalPopupExtender ID="ApproveCommentsPopup" runat="server" TargetControlID="btn_Approved"  backgroundcssclass="modalBackground" PopupControlID="pnl_Comments">
            
            </ajaxToolkit:ModalPopupExtender>
      </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField  HeaderText=" Reject " HeaderStyle-BackColor="Green" HeaderStyle-ForeColor="White">
        <ItemTemplate>
         <asp:ImageButton ID="btn_Reject" runat="server"  ImageUrl="images/Reject.png" Height ="25px"/>
          <ajaxToolkit:ModalPopupExtender ID="RejectCommentsPopup" runat="server" TargetControlID="btn_Reject"   backgroundcssclass="modalBackground" PopupControlID="pnl_Comments">
            
            </ajaxToolkit:ModalPopupExtender>
        </ItemTemplate>
        </asp:TemplateField>
           
        </Columns>
        </asp:GridView>