You own the copyright to your work, as with all work on Medium. And by entering, you are granting Medium non-exclusive rights, which means you can monetize your writing however you want, after the contest ends. Keep in mind the story can only appear on Medium (as in, can't be published elsewhere at the same time) during the contest period. Hope that helps clarify!

Content Lead for Writing @ Medium // Editor of Human Parts & Creators Hub // Novels: Unseen City; The Mermaid of Brooklyn; How Far Is The Ocean From Here