On an upcoming trip in Y, I am considering purchasing an extra seat to make sure there is extra room available. I notice that KLM has some information about buying an extra seat for passengers of size, where they say it qualifies for a 25% discount (and I imagine waived taxes). When emailing the TA they were confused by the request, instead talking about the OLCI extra seat option, which of course is not guaranteed until check-in happens. Instead, I'm looking to go via the route similar to the one specified for passengers of size to buy an extra seat.

Does anyone have experience on how to make this happen? Conversely, if I am ok in paying for buying an additional ticket as normal, does anyone know how to do this for a KLM flight? I imagine that normally reservations/tickets for the same passenger just get dropped off automatically.

EDIT: I am posting this in the general forum because even experience from Air France would be welcome. Just to have a data-point.