Answer by Mun Mun Das for FOSUserBundle Entity constuct

You have to modify onSuccess method of RegistrationFormHandler class. Check Overriding Form Handlers section of the doc. In your overridden method you just have to add

$user->setRefId($this->request->cookies->get('ref_id'));

Right before calling the parent's onSuccess method.

As for validation you have to create custom validation constraint where you have to inject @request and @doctrine.orm.entity_manager services to get the cookie and validate it across the database. You can check this cookbook entry.