I have a masterpage where a have defined a function below

 function PopUpShowing(sender, eventArgs) {
                var popUp = eventArgs.get_popUp();
                var scrollTop = (typeof window.innerHeight != 'undefined' ? 0 : document.documentElement.scrollTop);
                var popUpWidth = popUp.style.width.substr(0, popUp.style.width.indexOf("px"));
                var popUpHeight = popUp.style.height.substr(0, popUp.style.height.indexOf("px"));
                var windowHeight = (typeof window.innerHeight != 'undefined' ? window.innerHeight : document.body.offsetHeight - scrollTop);
                var windowWidth = document.body.offsetWidth;
                if (popUpHeight == "") popUpHeight = 300; // if the height isn't set on the popup, default to 300px 
                popUp.style.position = "fixed";
                popUp.style.left = (Math.floor((windowWidth - popUpWidth) / 2)).toString() + "px";
                popUp.style.top = (Math.floor((windowHeight - popUpHeight) / 4) + scrollTop).toString() + "px";
            }

Then i have i page that opens a radwindow on button click. The radwindow contains a radgrid that has a popup editing set. So i want to call the PopUpShowing function in the masterpage to display the popup.