Do you mean something like this Danny?
(defun c:prefix (/ pre ss idx ent obj)
(vl-load-com)
(and (setq pre (getstring t "\nEnter prefix: "))
(setq ss (ssget '((0 . "TEXT,MTEXT"))))
(setq idx -1)
(while (setq ent (ssname ss (setq idx (1+ idx))))
(setq obj (vlax-ename->vla-object ent))
(vla-put-textstring obj (strcat pre (vla-get-textstring obj)))
)
)
(princ)
)