You cannot order assistance for boarding the S-trains. If you want to travel with S-trains, you must place your wheelchair on the platform facing the front coach so you can signal the driver that you require help for boarding - and alighting if you tell them where you want to get off.
The Metro is designed to be accessible to all. The guiding philosophy is that mobility- as well as visually impaired people should be able to use the Metro with as little assistance as possible. On the station, there are e.g. wheelchair bays on forecourts, lifts from street level to platform, ticket dispensers (In a maximum height of 1200 mm), platform doors at tunnel stations and uniform floor surfacing with “guideways”. On the trains, there are entrances without steps and flex areas, increasing tone and light signal before the doors close, anti-trapping device on door edges, no “chair legs”, so there is space for guide dogs (but also dog free sections), and names of the stations are announced both over the loud speaker system and on information displays.