Any way to have a panel pop up on load?

Figured I would post this question here, after having all my attempts end in frustration. 

I need a panel to automatically open itself whenever the Xulrunner application is loaded and the window would pop up. 

I tried to do this with an onload command placed inside the window tag, but to no avail. Is there a proper way to achieve this? Other than binding the popup to a key and simulating a key press on load, that is?