The system will detect that you have already registered with a UV accountand will offer to link it to the University to assign you the free student license or the Overleaf Commons license if it is a PTGAS/PDI address.
If you registered using a non-UV personal account:
In Emails and Affiliations select "Add another email".
Add your official UV email address (@subdomain.uv.es) to link it to the University.
You will automatically be assigned your free student license or the Overleaf Commons license if it's a PTGAS/PDI address, and you'll be able to log in with your UV username and password.