You have to ask them to be added as a writer first, then, when you are ready to publish, there's a 'submit' option, which you can choose instead of publish. You can submit to any who have added you as a writer. Each publication has different guidelines, so it's a good idea to start with one or two, to get used to how it works. Most of them request unpublished drafts. But some will accept articles that you've self published. It depends on their guidelines.