I don't know why I can't use it because when I add the code is not working this is my code:
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default2.aspx.vb" Inherits="pages_frontEnd_Default2" %><!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"><head runat="server"><title></title></head><body><form id="form1" runat="server"><div> <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:TextBox ID="TextBox1" runat="server"></asp:TextBox><ajaxToolkit:NumericUpDownExtender ID="TextBox1_NumericUpDownExtender" runat="server" TargetControlID="TextBox1"></ajaxToolkit:NumericUpDownExtender></div></form></body></html>