Sending MIDI messages to Apple’s DLSMusicDevice

Hi, I’m wondering if anyone has an example they can share of how to send midi messages to apple’s internal synth. I’ve come across a few posts but none include any information on how to get this working. This post (MIDI port issue?) is from a while ago and has some info but I cannot find the actual functions that are being used. Could someone provide a basic explanation or provide some simple code so I can begin to understand how to hook all this up? Any help is greatly appreciated!

I figured out how to send midi messages to DLSMusicDevice by creating a plugin instance of the AU in my plugin and then calling its processBlock() method. It works great when running the vst3 plugin in Reaper but the standalone plugin crashes at this line:
jassert (isPositiveAndBelow (channelNumber, numChannels));
in juce_AudioSampleBuffer.h. I tried tracing the issue and I think it has to do with DLSMusicDevice having 4 output channels (2 stereo pairs) and the standalone only having 2 output channels (1 stereo pair) but I’m not sure how to resolve this. Has anyone come across this issue before?