The type of writing you’re describing is often referred to as "creative writing prompts" or "story starters." In educational settings, it may also be called "continuation writing" or "closure writing," where students are given an incomplete passage and asked to write a suitable ending. This exercise encourages creativity and helps develop narrative skills.

If you're just writing a few words, it could be a "fill-in-the-blanks exercise." But when you use the word "passage", it makes it sound like it might be more extensive. Did you mean "sentence" or "paragraph?" If you're writing more than just a few words to complete a sentence, it's simply a "writing exercise."

Hi Viola. I think it’s called continuation writing. Not an easy skill. Are you doing it?