ASP.Net AJAX ModalPopupExtender Modal Popup re-opens on browser refresh (F5) in ASP.Net

Hi, I have to show customise popup message on the successful insertion of record.So why I have used ajax ModalPopup extender control and have called this from aspx codebehind. It works conditionally good. But as I press F5 then it reloads again. Though this time I have not inserted any record so that it can call show() method of ModalPopup. Kindly help me how to prevent reloading this popup itself.

  <asp:Label ID="lblModalAlert" runat="server" style="display:none"></asp:Label>
                    <asp:ModalPopupExtender ID="modalpopupalert" runat="server" PopupControlID="Panel_alert" BehaviorID="mpe" TargetControlID="lblModalAlert"  BackgroundCssClass="modalBackgroundalert"></asp:ModalPopupExtender>
                    <asp:Panel ID="Panel_alert" runat="server" style="display:none" >
                        <asp:UpdatePanel ID="updatePanel_Alert" runat="server" UpdateMode="Conditional">
                            <ContentTemplate>
                                <div>
                                    <p id="alertParaID" style="text-decoration-color:red;">
                                        Thank you for your interest in our company. Our executive will contact you shortly.
                                    </p>
                                   <%-- <button id="btnalertPop" type="submit" onclick="javascript:hideModalPop();">OK</button>--%>
                                    <asp:Button ID="btnOKalert" runat="server" Text="OK" OnClick="btnOKalert_Click" />
                                </div>
                            </ContentTemplate>
                        </asp:UpdatePanel>
                    </asp:Panel>

code behind

 protected void btn_VSubmit_Click(object sender, EventArgs e)
        {
           // modalpopupalert.Hide();
            InsertVendorEnquiryRecord();
            modalpopupalert.Show();
        }

        protected void btnOKalert_Click(object sender, EventArgs e)
        {
            modalpopupalert.Hide();
        }