-BrowserFrame- how to set destination url

Hello, a simple question.
I created a BrowserFrame like this :

Browser sample = new BrowserFrame(“browser”, new ExternalResource(

and this frame go to example.com as first page. How i can set the url for second navigation?
I created a address bar and a button, but with “sample.setSource( url )” i am not able to change the url of BrowserFrame:

public void buttonClick(ClickEvent event) {

Thank you for help,