Like Chris said, you will have to use something like Memberstack - unfortunately you can’t just do it with Webflow forms, unless you’re accepting signups for a platform hosted somewhere else and are only using Webflow for signups. If that’s the case, then what you described is possible
Did you manage to solve this? We are struggling with the same issue, we are using the Memberships login page and created a dashboard, but we need to integrate the login/signup information with a 3rd party app and cannot trigger anything from Flows.