A signature line is a horizontal line aligned with adjacent text.
Typography purists avoid accomplishing anything by holding down keys on the keyboard. But in this case it’s the simplest solution. To make a signature line, hold down the underscore key (shift + hyphen) until you get the length you need.
Same thing, right? Not quite. There are three good reasons to prefer underscores to underlined word spaces.
If you need to quickly excise underlining from a document, you might want to select all the text and then uncheck the underlining option. But this will wreck signature lines made out of underlined word spaces—they will disappear.
If you need to quickly ensure you only have one space between sentences, you might want to search and replace double spaces. But this will also wreck signature lines made out of word spaces—by partially deleting them.
Underscore characters don’t depend on formatting, so they will look the same no matter where they’re copied and pasted. Underlined word spaces may not.
You should depart from this rule only if the font you’re using has an underscore character that doesn’t form a solid line when used in sequence. That’s how underscores are supposed to work. But some fonts are goofed up. If you see gaps between the underscores, either use underlined word spaces, or use the underscores from a different font. No one will know.