The A/B buttons on the Harmony M unit.
This puts it into MIDI channel set mode.

As I said it defaults to channel 1, but use the increment dial to change channels in the display (1-9 and then the same but with a dot next to it for the teens) whilst playing the keyboard section of the Yammie you want to use (assuming TX is on) just wait until you see some flashing on the H-M...then set that as the channel

Dennis