Sending midi to an external keyboard:
Track routing window -> midi hardware output
For voice control on an external keyboard:
Simple way: While recording, but prior to performing the music, use the keyboard to change to the voice you want. It will get recorded as a program change as part of the midi stream. So when you play it back, it will send that back to the keyboard and set it to the voice you want.
There are more sophisticated ways to do this, but try simple first.
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