Here's a "friend link" (ie non paywalled) for the submission guidelines: https://medium.com/reviewsday-tuesday/reviewsday-tuesday-submission-guidelines-be2b758adb4a?sk=13920cc23ad3a4ca207c42562b8206d7

and the monthly theme: https://medium.com/reviewsday-tuesday/reviewsday-tuesday-monthly-theme-june-2023-9244943e975b?sk=66aaa557ec1ad010067be79587412844

You don't have to be a paying member to write for publications, but it does sometimes help to drive engagement (since a lot of Medium is about reciprocity; people read you when you read them).