In GNU screen, the shortcut Ctrl-A + Ctrl-K or just K is mapped to kill the current window. I was wondering is there a shortcut to clear the current characters input so far in the prompt, just like the bash Ctrl-U or Ctrl-A + Ctrl-K would. I haven't been able to find it in the documentation. 
How to delete current line in GNU-screen (bash equivalent of Ctrl-K)?
gnu screen
If by prompt you are referring to the GNU screen prompt by Ctrl-A and :, that gets cleared by Ctrl-U.  That is standard for most command-line applications; the others are bash-specific.