you can prompt by creating a new "sign up" in the AO3 collection (this is a completely separate process to signing up for the fest as a writer. this is just what ao3 calls prompting). you can find a direct link here: https://archiveofourown.org/collections/gmgfest/signups/new

sorry for the late response! you do need an ao3 account in order to post prompts to the collection.