Pass email value to sign up page

In some use cases like marketing outreach or pre-signup questionnaires, the client app may want to pre-fill the signup page with the email/phone the end-user might use for a smoother user-experience.
Allow client to pass an identifier when sign up is triggered.

No files, tests, or entry points are named. Trace how the client triggers sign-up and how the sign-up page receives initial values, then determine how an email or phone identifier should be passed through. Done means the client can provide that identifier and the sign-up page pre-fills it.

Written by the indexing model from the issue text.