;; test.scm
(define-syntax test
  (lambda (e . _)
    (apply (lambda (x y #!optional (z "z"))
	     `(string-append ,x ,y ,z))
	   (cdr e))))
(print (test "x" "y"))
(print (test "x" "y" "z2"))
--
csc test.scm && ./test

>> Error: during expansion of (test ...) - unbound variable: optional