I can only get this to work, when I use an instrument from the old GarageBand '11, version 6.0.5.
As I had previously GarageBand '11 installed, all old instruments are still available for me in the current GarageBand 10.3.5 and I can use them by picking the instrument "Legacy > GarageBand > SoundEffects > Nature Sounds" for a software instrument track. Then I can drag samples from the Finder onto the keys of the Musical Typing keyboard. But not for the piano instruments installed by GarageBand 10. It is a bit confusing, because the Musical Typing may be showing the title "Classic Electric Piono", even of the track is called "Nature Sounds" and is a Sound Effect instrument track, not a Piano track.

If you never owned GarageBand '11 or the old instruments have become uninstalled, you can try to reinstall them from MainStage 3. when I purchased MainStage 3 a few years ago, it included all old GarageBand '11 instruments and Loops for download, plus all old JamPacks.