How do I add custom fields to the signup/login widget?

When using Classic Universal Login:

A signup screen can use the additionalSignupFields option in The Auth0 Dashboard -> Universal Login -> Login. Each option added in this manner will then be added to that user’s user_metadata after a successful signup.

A login screen on the other hand, cannot contain added inputs. An alternative is to request the user to provide additional information after they sign up or log in.