https://docs.juce.com/master/tutorial_handling_midi_events.html#tutorial_handling_midi_events_keyboard_state:~:text=functions,-.
void handleNoteOn (juce::MidiKeyboardState*, int midiChannel, int midiNoteNumber, float velocity) override
//Called when one of the MidiKeyboardState's keys is pressed.
{
if (! isAddingFromMidiInput)
{
auto m = juce::MidiMessage::noteOn (midiChannel, midiNoteNumber, velocity);
m.setTimeStamp (juce::Time::getMillisecondCounterHiRes() * 0.001);
postMessageToList (m, "On-Screen Keyboard");
}
}