You can use CMD+Shift+W to close the currently active (non-plugin) window, in your case Lyrics/Chord Pro window.
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Alternatively, you can use a button widget in the Global rackspace, that will toggle this window (can also be MIDI mapped, like any other button widget).
Give the button the GPScript name ''BUTTON" in the Widget Properties Advanced tab, and then paste the below code into the Global Rackspace Script Editor (Window menu). Click Compile at the bottom before closing the script editor window.
var BUTTON : Widget
On WidgetValueChanged (newValue : double) from BUTTON
if newValue == 1.0 then
ShowChordProWindow(true)
else
ShowChordProWindow(false)
end
end