Add underline to link in navigation bar

[mattdeo]

My Shopify site preview is https://dn29xmgs2ju2y6ra-23625195.shopifypreview.com.

I want to add underline to the link in the Navigation Bar that says "All Current Designs".

I want to do this with the Custom CSS box in the Customizer for the header bar. How do I do this?

[Made4uo-Ribe]

Hi @mattdeo 

Where can I find this "All Current Designs"? Its not visible on your header menu. 

[Dan-From-Ryviu]

Hi @mattdeo 

Please add this code and check if it works 

a[href="/pages/shop-proco"]:after {
    content: '';
    height: 1px;
    width: 100%;
    background: #000;
    display: block;
}