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))  # ¯