Glossy effect on stroke

I have the following rounded rectangle

Need to make border glossy like below image

In order to do that, I've expanded this shape and created a ellipse with no stroke above button, with transparency/screen. This is what I got:

Now I need to get rid of the rest of ellipse, so the button will be transparent on upper boundaries. Inner part of button should be white, but whatever I put between button and ellipse should receive glossy effect. How to do that?