KeyCastr requires access to the macOS Accessibility API in order to receive your key events and broadcast the keystrokes you are interested in. On newer versions of macOS (10.15+) there is a new Input Monitoring menu under Security & Privacy within the System Preferences app, and KeyCastr will appear there automatically the first time you run it. Simply unlock this menu and check the box next to KeyCastr to enable it. On older versions of macOS, or if for some reason the app doesn't appear...
Application sharing software that allows individual windows to be replicated to other computers. Remote collaboration tool based on a modified VNC protocol. Its advantage is to keep some things, like email, private, while sharing other desktop apps.