You can't actualy do that, you can however open a page that is centered on the middle of the screen and that opens fullscreen that takes the window focus....

this will however really piss off people who do not want to use your app such as for pupup window advirtising and you will get your site black listed if this is your intent.

I see no legitimate reason to try and force a user to stay at a screen so although it is possible Im not giving you any help until you can give me one.