Unicode character equivalent to the underscore on top
What you seek is generally known as a "macron", "overline", "overscore", or "overbar". It exists at several Unicode code points; which one you want depends on whether you want a macron or an overline (the former being shorter), and whether or not you want a combining character.
From Wikipedia, an overline may be found at U+203E (spacing) or U+0305 (combining), and a macron at U+00AF or U+02C9 (spacing, and apparently identical in appearance), or U+0304 (combining).
You could try a web service to look up the unicode character you want by drawing.
Unicode character recognition
Some results:
Horizontal scan line-3: ⎻
Unicode hexadecimal: 0x23bb
Combining double macron: ͞
Unicode hexadecimal: 0x35e
Upper one eighth block: ▔
Unicode hexadecimal: 0x2594
Combining overline: ̅
Unicode hexadecimal: 0x305
How to do an overline?
... ¯\_(ツ)_/¯ ... I don't know.
Joke aside, here is a short way to generate them in code if you need them (you can adapt to any language, here in Python):
print(chr(8254)) # ‾
print(chr(175)) # ¯