if the myFunction returns False... then you would have to call it like this:
<button onclick="return myFunction();">Try it</button>

you could also do this (and not modify the myFunction):
<button onclick="myFunction(); return false;">Try it</button>

I've done that before... but it would also depend on whether the form was just a container for the button (and if that's the case it should be removed to be semantically right)

-tg