*scratch*.el

(defun foo ()
  (interactive)       ; To make it a command that can be bound to a key
  (string-rectangle   ; Call this function
   (region-beginning) ; First arg: START
   (region-end)       ; Second arg: END
   "ZZZ"))            ; Third arg: STRING
   

;; More idiomatically:

(defun foo (beg end)  ; Args will be passed by interactive below
  (interactive "r")   ; This is a command operating on the region
  (string-rectangle   ; Call this function
   beg                ; First arg: START
   end                ; Second arg: END
   "ZZZ"))            ; Third arg: STRING