this is my rad window code
<telerik:RadWindowManager ID="RadWindowManager2" runat="server">
<Windows>
<telerik:RadWindow ID="RadWindow1" runat="server" Width="1000" Height="400" Modal="true">
<ContentTemplate>
<table>
<tr>
<asp:Label ID="lblFavorite" runat="server" Text="Favorites: "></asp:Label>
<asp:TextBox ID="txtReportFavorite" runat="server"></asp:TextBox>
</tr>
<tr>
<asp:Button ID="btnOk" runat="server" Text="OK" Width="50" Height="25" />
<asp:Button ID="btnCancel" runat="server" Text="Cancel" Width="50" Height="25" />
</tr>
</table>
</ContentTemplate>
</telerik:RadWindow>
</Windows>
</telerik:RadWindowManager>
in aspx file a added a javascript function
<script type="text/javascript">
function openwin() {
window.radopen(null, "RadWindow1");
}
</script>
then on code behind i called that javascript function
ScriptManager.RegisterStartupScript(this, GetType(), "displayalertmessage", "openwin()", true);