How to define this function without onRelease?

1

I need the following script to run as soon the as the play head comes on the frame. Right now it is activated through an onRelease…need to replace that:

myButton_btn.onRelease = function() {
tweenBall(mx.transitions.easing.Strong.easeOut);
};
function tweenBall(easeType) {
var begin = -50;
var end = 100;
var time = 20;
var mc = ball_mc;
ballTween = new mx.transitions.Tween(mc, “_y”, easeType, begin, end, time);
}