Sorry for the delay. I was just starting my response when you pinged me above.
The SCLogin module doesn't have any method to register users or add your own fields right now. When you click the "Register" button, we simply redirect the user to the built-in Joomla registration form. SCLogin has no control over the Joomla registration flow and we have no option to add more fields or mark some as required/not required there.
I hope that helps answer your question, but if you have any others or need more assistance, just let me know.
Thanks,
Alex