How do I write "Under Score" in Apple Watch using scribble

Draw the horizontal line near the bottom of the scribble area.

If that does not actually give you an underscore, then rotate the Digital Crown to move through a selection of similar shaped special characters.

This article might be helpful as well

https://www.idownloadblog.com/2017/01/30/master-scribble-apple-watch/

It probably isnt possible, or if it is, it’s difficult to do so since an underscore isn’t used very often. You can send feedback to Apple on that here: https://www.apple.com/feedback/

If you draw the line at the bottom of the screen it will do _ and do it in the middle you get -

As bob pointed out it. I tried it does work.