Special Note heads, I think.
I am transcribing some music that has an occasional spoken word or sound effect on the score. In commercially printed music this is often depicted as a note stem with the note head drawn in as a small x. (The stem 'attaches' to the "x" in one of the corners - not the centre).
I have not yet discovered how MuseScore does this. Almost all of the answers to my questions with this product have been "of course, just do this . . . " and I am hoping this is the same.