I tried what you say, but it isn't my solution :( . I explain you why:
for example i set in my htmlbox the following url: http://localhost:13050/AAA/example.aspx
my htmlbox load that page and inside of this page I've a filter box. So i put the filter and i press a button to trigger the filter action (all those elements are inside the page loaded by the htmlbox). Well, the htmlbox correctly reload the filtered page, and now the new url will be
http://localhost:13050/AAA/example.aspx?PAR=bbb (i can see that from my browser with right mouse click on the htmlbox and click properties).
But now, if i reload the src property, remain the initial (http://localhost:13050/AAA/example.aspx).
But it's fundamental for me retrieve the new url!