I'm working also with ASWebAuthenticationSession, and prefersEphemeralWebBrowserSession does not affect to show or not the promp to save the password. Basically, as @ArcadianZ said, ASWebAuthenticationSession is not able to show the save password Alert/prompt asking the user to save the user/password in Passwords App. I'm trying to think why and maybe it's because the save prompt happens on the URL after when it's entered, and in this case it's a redirect to an app URL?