To me this question has little to do with QGC - it’s a general question “how do I add login functionality to a Qt app”.
Your best bet is to search on that topic. Then ask for help in the broader Qt community or Qt on Android community.
People in this community can help you with answers that are more specific to existing functionality. They are also more likely to help if you can show that you have tried a few things out and they didn’t work.