Jacquie, no, your app does not have to be in the app store. With Apple (or Google), you can actually send your app to us through email and Apple will provide us a link to install it even when not in the store. It doesn't need to be complete, but core features should be in place - make it real, not just a concept or prototype.
Suraya, it looks like devpost doesn't give me contact info unless you are a submitter (not just participant). Feel free to email my team here for questions, and they can also get a hold of me directly: email@example.com.