How can I insert a line break before the metronome marking?
Not sure you can natively.
But you can write the Tempo and metronome mark as system text using MusGlyphs, and hide the original ones.
Edit: use MusGlyphs-text for the tempo and MusGlphs for the metronome mark
Thanks.
Do I need to enter it as “Staff text”?
Please read carefully my previous reply:
How do I use MusGlyphs?

