Katie, it sounds to me like you can't make a reservation online, but that you can contact them and explain what you need, and they will help you do it that way. That is my understanding of what I read.
That said, it sounds like Stuart's suggestion to use TPE may be the way to go. I did a dummy booking through them just now to check it out, and it worked well. You create an account, then, as he said, go to book "Book a Seat (or bike) Reservation", and enter the information for the train that you're requesting.
It will then give you a choice of options for a seat reservation with preferences, such as aisle, window, facing forward, and so on. Once you do that, it will issue you a seat reservation and give you all the information.
Below is a screenshot of what I got after I booked it all. Obviously, this was all dummied up stuff, but you can see what it looks like and what you get.
https://photos.app.goo.gl/LhUX9oz9CsZ2Lmo97