Use ASP.Net Login control inside AJAX UpdatePanel in ASP.Net

You can add the login control inside the Update Panel like this.

Ref: Simple Form based authentication example in ASP.Net

<asp:ScriptManager runat="server">
</asp:ScriptManager>
<asp:UpdatePanel runat="server">
    <ContentTemplate>
        <asp:Login ID="Login1" runat="server" OnAuthenticate="ValidateUser">
        </asp:Login>
    </ContentTemplate>
</asp:UpdatePanel>