To write fill-in-the-blank sentences in Markua, you can put single underscores around a set of "ideographic space" text characters. (Basically, you're underlining a sequence of empty characters.)

To test this, copy and paste the following line into your book:

Lorem _　　　　　　　　_ ipsum

That will produce the following, when you generate your book:

To change the width of the blank, just delete or add more of the ideographic space characters, which you can copy and paste, even though they're invisible.

If you're interested in the subject of "empty characters", please check out this website, where we found the "ideographic space" character used in this article.