I do not think you are looking for the Signin Microflow because that only does this:
An anonymous user in a web shop adds some items to her shopping cart and then clicks the ‘Check out’ button. A sign-in screen appears, and the user signs in. The sign-in microflow is executed and transfers the shopping cart of the anonymous user to the signed-in user.
I think you are looking for an Autologin functionality. There is a demo project in the appstore:
https://appstore.home.mendix.com/link/app/107925/Magnus-Consulting/AutoLogin
There are also some forum post describing how this works.
Regards,
Ronald