From your question I can understand that you want the user to give the details in the Registration entity during signup, right? If yes follow the steps.
1. Create an anonymous user.
2. Create an edit page with the entity and set it as the default anonymous page.
3. Use call a microflow button as a signup button where you create account.
For more details I have attached an YouTube video in which I have learned how to create custom login with signup: https://www.youtube.com/watch?v=w4-DEiZVZTA
Hope my answer helps. Reach me out for any queries.
Here you are using non-presistable entity so, the entered datas will not be stored in database. while the user click to save those detail there you create a microflow in that microflow create a account for the account attributes map your registration entity values and commit. So by this you can retrieve username and email id.