<telerik:RadScriptManager ID="RadScriptManager" runat="server"></telerik:RadScriptManager>
    <telerik:RadAjaxPanel ID="RadAjaxPanel" runat="server" CssClass="RadAjaxPanel">
        <div>
            <telerik:RadComboBox ID="RadComboBox1" runat="server" AutoPostBack="True"
                >
                <Items>
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem1"
                        Value="RadComboBoxItem1" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem2"
                        Value="RadComboBoxItem2" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem3"
                        Value="RadComboBoxItem3" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem4"
                        Value="RadComboBoxItem4" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem5"
                        Value="RadComboBoxItem5" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem6"
                        Value="RadComboBoxItem6" />
                    <telerik:RadComboBoxItem runat="server" Text="RadComboBoxItem7"
                        Value="RadComboBoxItem7" />
                </Items>
            </telerik:RadComboBox>&nbsp;
            <telerik:RadTextBox ID="RadTextBox1" runat="server">
            </telerik:RadTextBox>
            <asp:Label runat="server" ID="Label1"></asp:Label>
        </div>
    </telerik:RadAjaxPanel>


Code Behind:

 private const string NotificationId = "pageNotification";

    protected void Page_Load(object sender, EventArgs e)
    {
        if (RadAjaxPanel.FindControl(NotificationId) == null)
        {
            RadNotification notification = new RadNotification();
            notification.ID = NotificationId;
            RadAjaxPanel.Controls.Add(notification);
            notification.Show();
            Label1.Text += RadTextBox1.Text;
        }
    }