You can get MuseScore 3.6 to enter notes below existing notes.

I've wanted to enter notes below existing notes. I didn't want to use the workarounds, ex. select a note, press Ctrl + arrow down. This is how to enter notes below existing notes.

Put the program in Piano mode with P. Put the program in Note mode with N. Select the note you want to use. Hold the shift bar and select piano keys below the current note. Voila it appears. Just wanted to share this trick.

I used it in the song below for measures 1-6.