If Garageband is implemented in a way that MuseScore can recognize as a MIDI I/O device, then select it for output in Edit / Preferences / I/O. Or, if it supports JACK, then you can install and use that. Otherwise, you'd need to write to Apple to request one or both of those features.
Comments
If Garageband is implemented in a way that MuseScore can recognize as a MIDI I/O device, then select it for output in Edit / Preferences / I/O. Or, if it supports JACK, then you can install and use that. Otherwise, you'd need to write to Apple to request one or both of those features.