It's very easy:
On the website
Commence the ticket purchase process. Remember, the ALSAcab service is currently available only in Santander and parts of Madrid inside the M-40.
When you get to “Customise my seat”, select the ALSAcab option.
A window will show, where you will be asked to enter the pick-up point, the postal code of the area and a mobile number on which to contact you.
When you complete the purchase you will receive a confirmation e-mail with your bus ticket and ALSAcab ticket.
Points of sale and Support and Sales Line
Request the service during the purchase process. You may also reserve it if you already have your ticket. Go to “Manage your tickets” on the website or our other channels, entering your booking reference number.
On the day before picking you up at your home, place of work or other point, we will contact you to confirm the place and time of pick up. If you wish to be picked up at a station, we will call you while you are en route to confirm the meeting point.