It would seem that an online booking is not possible.
Based on the text shown below, you must request an appointment per eMail:
From: Ministerio de Asuntos exteriores, Unión Europea y Cooperación
- select Consular services
- fill in required data:
Inside the resulting text you will find:
Place of presentation:
The application must be submitted in person, at the premises of the Consular Office. It is necessary to request an appointment by writing an email to [email protected].