Get email address from registration form
Why does getBodyParam('email') always return empty here?


Because that method is looking for data in POST, and after the redirect has happened, it's a new request, and POST data is lost.

If you want to do what you're looking for, the easiest way is to pass the email address along as a query string param (i.e. ?email=foo@bar.com), then the template can do craft.app.request.getQueryParam('email').