How do I assign a NOTE to a button?

Hello,

I want to assign a note to a button. I would like to set it up where I press a button and it plays the note B on the Cello Tremello sound that will continue until I stop it by pressing the button again (toggle). This will free me up to use both hands to play a piano part underneath the "hum" of the cello.

I have gotten as far as assigning the button to the cello sound I want. But I can't figure out how to assign the B note to the button. I have spent about two hours trying to figure this out, with no avail.

I have an Oxygen 49 midi controller and a notebook air running the MainStage 2.

Thanks,

Michelle