GB playback via USB MIDI out

Hi folks! Running GarageBand 10.3.4 on Catalina 15.5, I want playback/audio output to go through USB MIDI to the connected MIDI keyboard. MIDI input to GarageBand works fine. But no matter what I do, output is played back through the MacBook's speakers and not the connected MIDI keyboard. 

25 years ago, on a PC with an internal PCI MIDI card running Win95 and CakewalkPro, this was no problem. Simply open CakewalkPro, select the MIDI card for output, and viola! The fact that I can't figure this out on modern equipment makes me feel old. :-)

Any thoughts?