I've opened that link 3 times - in Spanish, Valenciano and English - and the word 'registro', or 'rigistro' doesn't appear anywhere on the page.
The link takes me to page titled, "Justificante Vacunal Covid-19 Comunitat Valenciana", with the information:
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"The vaccination receipt includes information on the vaccines received.
Enter the required data and after VALIDATE , verify the last 3 digits of the mobile phoneIf it is not correct MODIFY CONTACT DETAILS .If it is correct SEND SMS and the doses will be displayed."
The required data is:
- SIP number
- Date of birth
- SIP card issue date (if you have one)
- Validation code
For information, I've added a screenshot of the landing page from the link.