Button Animation - underline to fill

I am trying to figure out how to make a button like the ones in the top section of this site:  https://brightenmade.com.  It looks like text with an underline, but when you hover, the underline fills up the button vertically.  Anyone have any ideas about how to accomplish this with code?  I'm on 7.1.

I used to do this for a site. This is possible with almost buttons (with form/newsletter button, it is a bit complex). If you share link to your site, we can give code for your case.

@tuanphan thanks!  I didn't post a link because I have barely started working on the site so it's literally trash right now 😆 but here is the link -- dogfish-bagpipe-dgc7.squarespace.com.  I have a button in the first section under the headline, which is where I'd like to see this effect.

See the video