Just use the Browser object's 'Request fullscreen' action. Sometimes it only works in a user-input trigger like a mouse click, touch or key press.
Hello Ashley, thanks for the quick answer, I'm Odin girlfriend, we allready tried that, you can see it here:
http://www.ursulalopez.com/musicrunners/
but doesn't work properly (on Google Chrome)
PS: I'm posting this instead of Odin, because I can write urls and he doesn't