This is your source code:
<AjaxControlToolkit:ModalPopupExtender runat="server"
ID="ModalPopupThanks" TargetControlID="hythanx"
PopupControlID="pnlThanks"
CancelControlID="Btnthnxok"
backgroundcssclass="modalBackground">
<asp:Panel ID="pnlThanks" runat="server" CssClass="modalBackground " >
<div>
</div>
<div style="text-align:center "><asp:Button ID="Btnthnxok" runat="server" Text="OK" /></div>
This is your c# code:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ModalPopupThanks.Show();
}
}