I don’t think you can pass the values from the authentication widgets to a microflow. What you could do is create a non-persistent entity with two string attributes (for username and password) and use those to capture a users credentials. Then you can use a mf button to pass that non-persistent entity to a microflow where you retrieve your SAP data to authenticate a user. Once the authentication is passed, you still have to log the user in. You can check out the blog post below that walks through logging a user in.
There is also a module in the appstore for this but the blog has everything you need. I’ll link that as well.
Hope this helps!