Do you mean an automatic registration, bypassing the nromal manual form fill out?
There are at least two possibilities:
1. If originating from email, pass the URL parameters along the URL for the landing page.
The form captures the URL parameters to hidden fields.
A small JavaScript submits the form when one of those URL parameters, say, autosubmit=true
2. If totaly random, catering for users clicking email or just reaching the landing page through navigation
A JavaScript evaluates whether the cookie exists or not.
If it does, it redirects automatically to the relevant page or content;
If it does not, it redirects to the form.