Navigation Menu - Active underline of drop-down menu items?

Hello all,

Looking for some assistance creating an active underline under the drop-down menu items on my site: https://shuifeng.squarespace.com/

Currently the active navigation items on the main navigation menu are underlined as desired. The active navigation item in the drop-down menu is not underlined (when selected) although I would like it to be. Is this possible?

Thanks. 

Thanks so much Tuan. That worked beautifully.