I don't have a wallbox / charger, I just use the granny charger overnight.
I plug the car in with the granny charger, Octopus detect the car starting to charge and Immediately turn it off (via the app access and api - it's done automatically ). They then setup a schedule to suit their electricity costs, typically between 23:30 and 05:30 but sometimes outside this band (the house still gets cheap electricty during this period regardless of the car charging or not). They then activate / deactivate the charging via the app/api as required to meet their schedule until the car is charged to the %age level and time I provide in the Octopus app.
So all you need to do is plug the car in, Octopus does the rest. I've never used anything on the in car infotainment system related to charging.
They need either the car or the wall charger compatiblte with Intelligent Octopus, i.e. the ablity to control one or the other, in my case it's the car as they can't control the granny charger. I don't know what they do it both car and wall charger are compatible, I assume they do a similar thing if the wall charger is compatible and the car isn't.
So in summary I guess you need the Cupra AND the subscription app, or an IO compatible wall charger.