Js dynamically adds a drop down box data source to the textbox

 
$("#<%=lstCarBrand.ClientID%>").change(function () { 
$("#<%=txtCarSeries.ClientID%>").val("");//Clear text box
$("#<%=txtCarSeries.ClientID%>").unautocomplete();//Clear autocomplete
$("#<%=txtCarSeries.ClientID%>").autocomplete('Ajax/AjaxCusCarEssentialVersion.aspx', {//Add the autocomplete
width: 200, 
multiple: false, 
matchContains: true, 
extraParams: {//Passed parameter
"CarBrand": function () { 
return $("#<%=lstCarBrand.ClientID%>").val(); 
} 
} 
}); 
});