BackKey in browser widget

[hh]

(Assuming you don't mean the backKey that leaves your app.)

This is eventually not possible from security reasons.
There are brute force javascript approaches that probably will
break as soon as libbrowser changes and have side effects.

Using "on browserNavigateBegin" will lead to an infinite loop
as soon as you load images or any tiny other thing from server
pathes outside your public_html.

What you can do is not to use a URL but to set the htmltext.
Then there is no history and the browser back doesn't work.