Dropdown for menu heading – Twenty Fifteen

Using Twenty Fifteen child theme. I have a custom link for a menu heading, with sub items under it. There is a drop down arrow after the menu heading that works fine. I would like the menu heading itself to also drop down the sub items (not just the arrow). Currently it appears as a link, but doesn’t go anywhere. The url is #. Where can I find the url for the sub items? Or how to do this?

Thank you!