Since there is no reception on site that could carry out the standard process of checking your documents, it is necessary to register your stay remotely BEFORE arrival. Otherwise, it may not be possible to provide access to the facility, as failure to comply with the registration requirement is subject to a high fine for the owners of the facilities.
More information can be found on the website: https://www.boe.es/eli/es/rd/2021/10/26/933  
The application form in which we provide your data looks like this: (example)

The obligation to provide full and true information rests with the guests of the facility.
For security reasons, we suggest the following methods of providing the data required for registration:

• via the Booking.com messaging system,
• via WhatsApp (after registration, the message can be deleted).

Understanding your concerns about sending personal information, we declare that we do not store your data after registration, it only remains in the government system (Gobierno de España. Ministerio de la Presidencia Justicia y Relaciones con las Cortes).

We thank you in advance for your understanding and cooperation and ask you to provide as many sets of information as the number of people registered for the stay, no later than 5 days before the check-in date.