Hhmm, I'm not familiar with that extension....and I can't seem to find it at the link you gave. I'd like to try it though.
Oh, but I found this:
https://github.com/mcjohnalds/inkscape-underlinetextWithout that, you have to just draw the underline with the Pen or Pencil tool. Yeah, I know....it's a big ol' pain. I guess we just have to remember that Inkscape is not a word processor. Maybe version 1.0 will have it??? I really have no idea, but I hope.
I'll install that extension and post how I like it shortly. Oh wait, I already have it installed, haha. I must have forgotten, because I haven't tested it. Let's see....
Aaww.... Sadly, it won't underline single words. It can only underline the whole text object. That means, if you want one word underlined (or one sentence when the text object is several sentences, you have to type it as a separate text object, and go to a lot of pain to align it with the rest of the text.
If 1.0 isn't going to have text underlining, I might ask that extension author for a new feature....