Here is the sample code:
web.config
<add assembly="RadComboBox.Net2, Version=2.8.5.0, Culture=neutral, PublicKeyToken=175E9829B585F1F6"/>
Default.aspx
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default.aspx.vb" Inherits="_Default" %>
<%@ Register assembly="RadComboBox.Net2, Version=2.8.5.0, Culture=neutral, PublicKeyToken=175e9829b585f1f6" namespace="Telerik.WebControls" tagprefix="rad" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<rad:RadComboBox ID="RadComboBox1" runat="server" AllowCustomText="True"
MarkFirstMatch="True" SkinsPath="~/RadControls/ComboBox/Skins" Width="150px">
<Items>
<rad:RadComboBoxItem runat="server" Text="One" Value="One" />
<rad:RadComboBoxItem runat="server" Text="Two" Value="Two" />
<rad:RadComboBoxItem runat="server" Text="Three" Value="Three" />
<rad:RadComboBoxItem runat="server" Text="Four" Value="Four" />
<rad:RadComboBoxItem runat="server" Text="Five" Value="Five" />
</Items>
</rad:RadComboBox>
</div>
</form>
</body>
</html>