This code snippet demonstrates how to insert a javascript code overriding the ASP.NET default control handling code lbxExpVendor.Attributes.Add("OnChange", "ctl00$ContentPlaceHolder1$txtExpVendor.value =ctl00$ContentPlaceHolder1$lbxExpVendor .options[ctl00$ContentPlaceHolder1$lbxtxpVendor.selectedIndex].text;") lbxExpVendor.AutoPostBack = False