Thank you Patrick, but I'm already using the C1WebCommand ver 2.0.20073.170.
The alert command question appears but if I answer NO the C1WebMenu fires the postback, this is the problem. If the onclick event return false the postback must NOT be fired, in a standard button the onclick event work in this way.
I have a context C1WebMenu with a Delete menu item and I must ask client confirm to the command, if the user click NO the postback event must be cancelled (I think), or there is another way to do this ?