how to make a control relative to another control

I have a button that I need to have a panel open relative to this button. This panel opens as a modalPopupExtender. Can anyone give me some suggestions on how to do this ?
 <asp:Panel ID="pnLoginControl" runat="server">
    <asp:Login ID="lgLogin" runat="server" BackColor="#E5E5E5"
        BorderColor="#CCCC99" BorderStyle="Solid" BorderWidth="1px"
        Font-Names="Verdana" Font-Size="10pt" OnAuthenticate="lgLogin_Authenticate"
        TextLayout="TextOnTop" VisibleWhenLoggedIn="False">
        <TitleTextStyle BackColor="#6B696B" Font-Bold="True" ForeColor="#FFFFFF" />
    </asp:L ...