<telerik:RadNumericTextBox ID="radTxb_CIF" runat="server"
                 MaxLength="9" Width="150px" style="height: 22px">

protected void Page_Load(object sender, EventArgs e)
    {
       radTxb_CIF;

    }

protected void Page_Load(object sender, EventArgs e)
{
   radTxb_CIF.Text = myvalue; // sample assignment

}