Implementing a secure confirmation page for an entity form (D8)
In drupal there are several way to do the same thing.
What I suggest in this case (certainly different for another person) use a small custom module with Cookies or Session.


Create a custom module.
Add an extra submitter.


Save the new USER ID on session, cookie ...
Redirect to the page (Ex : $form_state-&gt;setRedirect('your.module.route.name');)

Create a small dynamic custom page (Like : http://www.drupal8.ovh/en/tutoriels/5/create-a-simple-module-for-drupal-8-step-2-create-simple-page)


Get the USER ID from session, cookie ...



You can change the confirmation mail template from back office.