Can't find how to add a long horizontal line in the lyrics to denote a sustained word.

At the end of a sentence in the lyrics, I want to show the word extends to the end of a slur or to the end of a series of slurs, by including a horizontal line in the lyrics, for the same width as the slur(s).

I tried using underscores or hyphens, but they don't get rendered. I also tried to use the horizontal line from the "Lines" pallet, but that doesn't do it. Is there a correct and easy way to do this? If so, how? If not, please add the feature.