The Windows RegisterHotKey API doesn't allow most modifiers to be set as hotkeys, because it steals the key from other programs. There is not much I can do about this. I think your best bet is to try using AutoHotKey instead, or try keys other than F24. (F12? I think F24 might be too high.)

If I wanted to make Ripcord handle any possible keyboard key as a push-to-talk global binding, I'd have to use Raw Input and listen to all keyboard inputs -- which makes Ripcord a bit like a keylogger. Since Ripcord's primary focus is being a chat client, and not a companion voice chat software for gaming, I'm hesitant to spend the time and danger to add this feature. Lots of people request this, though, so maybe I will end up doing it. There are other similar tickets that also have this request, so I'll close this one. But I'm definitely hearing you about this... maybe I'll make it an optional feature with a disclaimer about safety, or something.