> Uwe Brauer <oub@mat.ucm.es> writes:

> Try `message-beginning-of-header' instead. Alternately, fooling with the
> value of (variable) `message-beginning-of-line' while calling (function)
> `message-beginning-of-line'.

Great, I did

(defun my-add-subject-test ()
  (interactive)
  (message-goto-subject)
;  (message-beginning-of-line)
  (message-beginning-of-header 'subject)
  (insert "[Test] "))

And this work nicely for both, empty and non empty subject lines!

Thanks 😁