Depends on the route (and travel date). Give some more details and we’ll advise.

Most seat reservations are available through the railway companies directly → it’s even cheaper (no fees). First, have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm

Paris > Barcelona (Sat 26/4/25) and Barcelona > Madrid (Sunday 27/4/25)

Couldnt see anything specific about booking without a pass on seat61

I have sent an email yesterday to Eurail, and a follow up today. There is only one service that suits on each day and I am hyper aware of there not being many disability/wheelchair spots available.

Appreciate any help you could offer

For Paris - Barcelona Rail Europe is the best: I'm not sure you can specify wheelchair spots during booking though. It might be worth calling SNCF, most staff are helpful on the English line.

https://www.sncf-voyageurs.com/en/contact-us/our-contact-points/by-phone/

Barcelona-Madrid might be more troublesome: reservations are available on the Eurail website but no wishes possible. Not available on the RENFE website and the helpline doesn't provide reservations either. If by chance you have a friend living in Spain, they could go to a counter and book there…

(If someone has better ideas, feel free!)

Maybe ​@Schelte knows what you can do/who to contact to reserve a wheelchair space in Spain?

In France station and train staff is very helpfll + you get absolute priority skipping the queue’s, but you need to call the phone number ​@thibcabe mentioned.

Travelling & sitting in your own wheelchair on AVE trains is possible for just €7, no pass required. On their main booking engine, indicate (under “more search options”) that you want a reduced mobility place (Plaza H - it's really just some space to fasten your wheelchair, so not a seat), select an available train and under passenger details > accessibility tick the box “request assistance” and select from the dropdown “Viaja en su silla de ruedas (Plaza H)”. The fare should change to €7. For the companion, I’m not sure how to get a seat near the wheelchair space, as the booking options for passholder fares generally don't allow seat selection. Maybe reach out to Renfe or to one of the travel agencies offering reservations. There are very limited “plaza H” spots available per train, so book on time!

If not travelling in your own wheelchair, assistance can be requested (up to 30 minutes before departure in Barcelona) through their dedicated app. Buy a regular seat reservation through one of the channels (either interrail reservations portal or happyrail). I don't expect these regular seats to sell out in the coming weeks for the end of April, so no worries in that case.