Keep in mind that the url can be a function that returns the URL as a string. So if it is easier to code, you could also do something like:
var dataSourcestreetsDDL = new kendo.data.DataSource({
transport: {
read: {
url: function () {
var searchAddress = ...;
var searchString = ...;
return searchAddress + "townName=" + searchString + "&f=";
}
...
The function will be called whenever the datasource needs to read data. It might be easier than changing things when the dropdown closes.