There is now way you can change this URL. It is built by the browser.
You could change the URL by javascript before sending the request but the better way is to use the post redirect get pattern (PRG).
You first post to the controller and redirect to the url that you build in the controller. That way you have full controll over the URL.
EDIT
this is a sample
public ActionResult FilterResult()
{
RouteValueDictionary searchRoute = ControllerContext.RouteData.Values;
if (searchRoute["Filter"]==null)
{
searchRoute.Add("Filter","");
}
searchRoute["Filter"] = "filter1,filter2,filter3";
return RedirectToAction("Search", searchRoute);
}