Time signature with rhythmic denominator ("Tempo ritmico")
MuseScore offers the possibility to create a Time signature or even Additive (composite) meters.
For the need to have an indication of (in italian) 'Tempo ritmico' currently you can use a Workaround: Shift+T to delete the value
but since it is not possible to replace it, e.g. with metNoteQuarterDown,
the desired symbol can be added via Master palette then you need to adjust the position with Inspector to get
The combination, currently, can not be added to a custom palette for future use.