How to define a command that works on the selected region in Emacs Lisp?

(defun command-on-region (beg end)
  "..."
  (interactive "r")
  (message "beg: %s\nend: %s" beg end)
  (deactivate-mark))

(global-set-key (kbd "C-<f1>") #'command-on-region)