Asynchronous communication deliberately slows down communication to make it more thoughtful. If you slow down communication, though, you need to give people time to respond. If you’re a product owner, give your dev team a chance to see the stories before the sprint starts. Queue up candidate stories for the next sprint, about a week in advance. Invite devs to look at stories they'd like to sign up for so they can drop questions which the PO, TL or tester can respond to, inline. In fact, this Q&A can enrich a story description and further reduce the need for a story kickoff.