Switch screen if only specified key is hold

I've seen similar questions at other sites, but I've seen no answers.

I want Synergy to switch screen if only I press some key (say, Shift), and move mouse to the edge. Without pressed key, Synergy should not switch.

I've examined all the options here, still no answer.

I know there is an option keystroke(key) = action, so, the only suggestion I've seen is to define some hotkeys (say, Ctrl+Alt+Shift+Left to switch to left display, and Ctrl+Alt+Shift+Right to switch to the right display. But this is NOT what I need, this actually isn't convenient: I should leave the mouse, put both hands on keyboard and press this hotkey.

The most convenient way to prevent accidental switches is to set up a key that should be hold to make switch possible.

I really hoped that we have options keyDown and keyUp, then I would do something like:

section: options
  keyDown(Shift) = ; lockCursorToScreen(false)
  keyUp(Shift)   = ; lockCursorToScreen(true)
end

But, unfortunately we don't have these options. NOTE: we have actions with these names, but not options. So, the code above doesn't work.

How can I achieve this?