How to add a beam over a rest
- Enter a 8th note, followed by a 8th rest, followed by a 8th note.
- Select the rest.
- In the "Beam Properties" palette, click the second icon: "Middle of beam" (double-click in versions prior to 3.4).
The video shows it with MuseScore 2, but it works that way with MuseScore 1 and 3 too.