How to center popup from ActionScript without using onclick?

[#scb]

I have a web page that contains a Flash piece which uses JavaScript to launch a popup window from the Flash piece. It works fine, but I would also like the popup to launch centered on the page. How can I do this from Action Script code without using onclick? (Or can I use onclick in ActionScript? I am pretty new to ActionScript.)

The JavaScript is:

function popflash() {
flashwindow = window.open("theurl.html","flashwindow","width=450,height=450,menubar=no,toolbar=no,status=no");
}

And the current ActionScript code is:

inv_btn.onRelease = function(){
getURL("javascript:popflash()");
};