Keep modifier keys held down

I am trying to use Witch ( as my application switcher on CMD+Tab. However when I set CMD+Tab to launch Witch BTT releases the CMD key when I lift off tab. I want it to keep the modifier key held down and work like the application switcher option.
Is it possible to have a key sequence where the modifier keys do not send a key up until the modifier is actually released?

Hi - any update on this? I would also love this

I also need this

Please consider voting for my feature request: Support keeping modifier keys down until released