I am also working on a similar task. Once the username, password and Signin buttons are added to the page, the SignIn button cannot call a Microflow. I am not sure what the next step is from here.
How to get this username and password to a REST POST call from here? Is there any example or documentation on this?
Have a look at https://docs.mendix.com/refguide/authentication-widgets
Mendix provides a nice demo in every app you create using Studio implementing MendixSSO, adding “MendixSSO_AfterStartup” to the project's startup-routine.