Insert variables from script into HTTPService url

Question

I'm trying to insert values from script into a HTTPService
url. How do I go about doing this? I tried the code below, but it
gives an error. Can anybody help?
<mx:HTTPService
id="httpCall"
url="
http://api.service.com/user/{userName}/list?count={apiCount}&offset={(apiOffset)}&sort={ap iSort}&appkey=1234"
resultFormat="e4x"
result="callHandler(event);"
fault="httpFaultHandler(event);"
showBusyCursor="true"
/>
<mx:Script>
<![CDATA[
import mx.controls.Alert;
import mx.rpc.events.FaultEvent;
import mx.rpc.events.ResultEvent;
[Bindable]
private var userName:String = "myName";
private var apiCount:uint = 10;
private var apiOffset:uint = 0;
private var apiSort:String = "desc";

Answer 1

Set the HTTPService properties in an AS function.
<mx:HTTPService id="httpCall" resultFormat="e4x" ...
result="callHandler(event);" ...../>
private function getUserData():void
var sURL:String = "
http://api.service.com/user/"
_ userName + "/list:;
var oRequest:Object = {count:apiCount,offset:apiOffset,
sort:apiSort, appkey:"1234" }
httpCall.url = sURL;
httpCall.send(oRequest);
Tracy