It would seem that an online booking is not possible.

From: [Ministerio de Asuntos exteriores, Unión Europea y Cooperación](https://www.exteriores.gob.es/en/Paginas/index.aspx)
- select [Consular services](https://www.exteriores.gob.es/en/ServiciosAlCiudadano/Paginas/Servicios-consulares.aspx)
   - fill in required data:
      - [Consular services, San Francisco, United States, Schengen Visa](https://www.exteriores.gob.es/en/ServiciosAlCiudadano/Paginas/Servicios-consulares.aspx?scco=Estados+Unidos&scd=251&scca=Visados&scs=Visado+de+estancia+(visado+Schengen))   

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]**.