How do I access the ModalPopupExtender that lives on the MasterPage from a content Page?

<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server" >
            <asp:Panel ID="pnlNewCustomer" runat="server" Style="display: none">
                <asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="False" 
                    DataKeyNames="CustomerID"  DefaultMode="Insert" DataSourceID="ldsCustomer" Height="50px" 
                    Width="125px">
                    <Fields>
                        <asp:BoundField DataField="SecurityID" HeaderText="SecurityID" 
                            SortExpression="SecurityID" />
                        <asp:BoundField DataField="CustomerCompanyName" 
                            HeaderText="CustomerCompanyName" SortExpression="CustomerCompanyName" />
                        <asp:BoundField DataField="CustomerAddress" HeaderText="CustomerAddress" 
                            SortExpression="CustomerAddress" />
                        <asp:BoundField DataField="CustomerCity" HeaderText="CustomerCity" 
                            SortExpression="CustomerCity" />
                        <asp:BoundField DataField="CustomerState" HeaderText="CustomerState" 
                            SortExpression="CustomerState" />
                        <asp:BoundField DataField="CustomerZip" HeaderText="CustomerZip" 
                            SortExpression="CustomerZip" />
                        <asp:BoundField DataField="CustomerPhone" HeaderText="CustomerPhone" 
                            SortExpression="CustomerPhone" />
                        <asp:BoundField DataField="CustomerContactName" 
                            HeaderText="CustomerContactName" SortExpression="CustomerContactName" />
                        <asp:BoundField DataField="CustomerContactEMail" 
                            HeaderText="CustomerContactEMail" SortExpression="CustomerContactEMail" />
                        <asp:CheckBoxField DataField="CustomerIsActive" HeaderText="CustomerIsActive" 
                            SortExpression="CustomerIsActive" />
                        <asp:CommandField ButtonType="Button" ShowInsertButton="True" />
                    </Fields>
                </asp:DetailsView>
                <asp:LinqDataSource ID="ldsCustomer" runat="server" 
                    ContextTypeName="MyDatabaseDataContext" EnableInsert="True" 
                    TableName="Customers">
                </asp:LinqDataSource>
                <div>
                    <asp:Button ID="btnDone" runat="server" Text="Done" />
                </div>
            </asp:Panel>
            <cc1:ModalPopupExtender ID="mpeNewCustomer" runat="server" DynamicServicePath=""
                Enabled="True" PopupControlID="pnlNewCustomer" TargetControlID="pnlNewCustomer"
                BackgroundCssClass="modalBackground" DropShadow="true" CancelControlID="btnDone">
            </cc1:ModalPopupExtender>
        </asp:contentplaceholder>
 
 
 
 protected void Button3_Click(object sender, EventArgs e)
    {
       cc1:ModalPopupExtender
        var cust = Master.FindControl("mpeNewCustomer");
       
    }

                                  

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