Flash AS3 - How to pass a value form Search Form to URLRequest string?

programming flash
1

How can I replace the value in the getstring for parameter partNumber=a100 with whatever the user typed into the search box.

http://www.stockmarket.aero/StockMarket/SearchActionR.do?partNumber=a100&communityName=BLUE&partial=true&theAction=search

AS3 Code

var link:URLRequest=new URLRequest(“http://www.stockmarket.aero/StockMarket/SearchActionR.do?partNumber=a100&communityName=BLUE&partial=true&theAction=search” + search_txt.text);
search_btn.addEventListener(MouseEvent.CLICK, onClick);
function onClick(event:MouseEvent):void
{
navigateToURL(link);
}