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Anyone know how to match this bottom nav element design using Javascript? (should be simple)

Hi there, I am trying to figure out how to get the bottom black line that follows whichever nav element you are currently on from this site:

This nav bar animation would be using JS with popups on screen. So all of this would take place on the same on-page in Webflow. Thanks in advance!

Here is my site Read-Only: LINK
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