[O] Highlight text in org-agenda-prefix-format

[Abdo Haji-Ali]

I want to highlight certain text in my todo list in org-agenda.
I thought I could do something like this

  (defun my-func
        (propertize "texty" 'face 'org-warning))
  (setq org-agenda-prefix-format '((todo .  "%(my-func) ")))

But the text appears without any formatting. Is there a reason for this?
Is there another way to highlight text in the todo-list in org agenda?