Set Ctrl+Shift as prefix in tmux

Ctrl and Shift are modifiers. These keys aren't transmitted to applications running in a terminal. Rather, when you press something like Ctrl+Shift+A, this sends a character or a character sequence at the time you press the A key. See How do keyboard input and text output work? for more details.

There may be some terminal emulators that can be configured to send a key sequence when you press Ctrl+Shift, but even that isn't a given and might depend on which order you press the two keys in, and you'd lose the ability to make Ctrl+Shift+key shortcuts.

If your terminal emulator permits it, you could configure it to send C-b a when you press Ctrl+Shift+A and so on. That would allow you to use single-keychord bindings for some commands.

If you want to free the keychord Ctrl+B so that it's sent to the underlying application, pick a different prefix such as C-\ or C-] or C-^.

See the man page for the list of keys tmux recognises:

KEY BINDINGS
When specifying keys, most represent themselves (for example ‘A’ to ‘Z’). Ctrl keys may be prefixed with ‘C-’ or ‘^’, and Alt (meta) with ‘M-’. In addition, the following special key names are accepted: Up, Down, Left, Right, BSpace, BTab, DC (Delete), End, Enter, Escape, F1 to F20, Home, IC (Insert), NPage/PageDown/PgDn, PPage/PageUp/PgUp, Space, and Tab. Note that to bind the ‘"’ or ‘'’ keys, quotation marks are necessary.

Note that Shift is not included...