(defun my-edit-indirect-prepare ()
  (setq header-line-format
        (substitute-command-keys
         "Edit, then exit with `\\[edit-indirect-commit]' or abort with \
`\\[edit-indirect-abort]'")))

(with-eval-after-load 'edit-indirect
  (add-hook 'edit-indirect-after-creation-hook
            #'my-edit-indirect-prepare))