Appointment at police for issue first DNI (Madrid)

How can I fix an appointment at a police station in Madrid for issuing my first DNI certificate? The web form for reserving an appointment requires for log-in filling my DNI which I don't have yet. Does the system legally support a reservation by other citizen that has already DNI number and asks reservation for appointment on my behalf? I am asking because I cannot log-in the online platform and read if such option is supported. Or - is it possible to do it by fax directly to the police station? Or- can I show up without an appointment. Will they consider it that I am coming to Madrid from abroad for short visit ?