How do I add a new segment to the spacemacs mode line?

init.el

  (spaceline-define-segment foo
    "Foo Docs"
    (when (bound-and-true-p spaceline-foo-p)
      "Foo: TODO")
    :when active
    :global-override foo)
  (add-to-list 'spacemacs-spaceline-additional-segments '(foo))
  (spaceline-spacemacs-theme '(foo))