seat61.com says:

Once you've bought your passholder ticket, you can choose better seats from a seating plan like this:  Go to www.eurostar.com, click Manage booking, enter your booking reference & last name, when your booking appears click Change seats and select your preferred seats from a Eurostar seating plan. The Eurostar website will email you another ticket showing your new seat.

Have you tried that?

Thank you.  Yes, I did that.  Instead of saying “change seat”, it says “change meal”.  There is nowhere to change the seat.  For our next seat reservation bookings (2 more on Eurostar), I would think if I made the seat reservation direct with Eurostar vs on Eurail site, I would have this opportunity to change seats.  But Eurostar asks for Cover Number in order to make the new seat reservation, and I don’t have one because it is a mobile pass.  I’m just going around in circles!  I think for the reservation I already made, we are not able to change seats and just have to live with whatever the computer assigned us!  It’s rediculous!  And, I don’t know how to make it work on the next two bookings.

So unfortunately it looks like Eurostar have taken away this possibility, or they're having a problem on their website. I've asked Seat61 if they know anything about it.

If you've contacted Eurail customer support for pass cover numbers, they will supply them. They give them to any mobile pass holder who asks.

Thank you.  I’ve seen examples of how it SHOULD look, but where the change seats link should appear, it says change meals, which makes no sense.  I think it is because we booked through Eurail rather than Eurostar direct.  I hope we are able to get the numbers from Eurail.  That is not proving to be easy either.  I wonder if it makes sense to even try to book direct with Eurostar or just do it through Eurail and take whatever seats they decide for us.  In other words, give up and give in.

The man in seat61 replied to me that it did work for him earlier this year (for a reservation booked through Eurail) but now not anymore for him either. He doesn't know why.

It’s really a shame they make the process so difficult.  And, frustrating.

Yeah it is the best is to write a email to the Company (Eurostar) and maybe if they get enough of them they reinstall this service.

@rvdborgt 
Maybe a spreatsheet, note file with access from Seat61,you and few other ticketing experts about services that changed (Currently i notice that i have not enough time to be uptodate as iam in Exam weeks )

Hi btravel, Unfortunatly Eurostar is temporarily redirecting Eurail and Interrail pass holders to Eurail for reservations. As for the pass cover number, one has been provided via a private message. Cheers,

Interesting - do you have any idea when they plan to restore that possibility on their own website?

This is really confusing.  So, there is no way to make the reservation with Eurostar directly?  Which means there is no way to select seats (for which they charge extra) on a train, since Eurail doesn’t allow it?  I’m thinking I would probably have been better off not getting a Eurail Pass, and just purchasing tickets.  I got the Pass Cover Number, thank you.

I am still very unclear on how to book eurostar and apply the pass code as there is no option on either site for this and both sites direct you to the other. I have to agree with btravel that eurail certainly seems to be far more trouble and hassle than it is worth

Online booking possibilities for Eurostar are currently the Eurail website (booking fee 2€ per person and train) and the B-europe website (booking fee 4€ per order).

You can also call NS or NMBS/SNCB (no booking fees but the NMBS/SNCB number is more expensive).

When I go to the B-europe site, there is no option to enter eurail pass code. It just goes right to charging my credit card 190 euros no less!!

Did you follow the link I posted? It looks like you didn't.

yes. I had already been on the site and then followed your link in case it was different. Once you find a ticket, it just leads you to payment. As I have said, there is no option to enter the pass 

The link to b-europe above is correct and you should come to the following page.

yes and the only option it gives is to enter what class pass you have, There is no where to enter the actual number

You will have to enter the pass cover number when you order. This booking website is specifically for pass holders only and, for Eurostar, wil only show trains with 30/38€ (2nd/1st class) pass holder seats available.

That makes no sense to me. I have a eurail pass which I paid for. I was told I can use that to book eurostar. I go to eurostar and it give me no option to enter my pass number and I am being asked to pay 190 EU. It is not showing me 30/38EU options. This is beyond frustrating

You cannot book pass holder reservations on the Eurostar website. That used to be possible, but Eurostar removed that possibility. It's no use trying to book on Eurostar.com: it won't work.

As I explained above, the remaining online booking options are on the Eurail website (log in to see availability) and the B-Europe website.

@jessicasaum 

If you read the answers and follow the instructions you will succeed in making the reservation.

You must do the reservation through the b-europe link provided above, not on the Eurostar website.

If you have a mobile pass you must start by getting your pass cover number from the PCN generator. The PCN generator is available on the Community website. If you have a paper pass, then the PCN is at the back of your pass cover.

There must be some communication barrier as I have attempted to book the ticket just as you advise above. I know I have to book through b-Europe and I have obtained my pass number. When going to book on the b-Europe site, as I have said, there is no option to enter the pass at any point in the process. There is only the option to enter the TYPE of pass which I have done and at that point, the next step is billing at a cost of 190 Eu. I am not sure what else to say as this is the reality

Ok. What route, date and time are you making the reservation for? How many persons? Interrail or Eurail? 1st class or 2nd class?

Aug 31 paris to London. I have a eurail pass and am attempting to book via b-Europe as I can not book on eurai. 2nd class pass

I don’t know what you are doing but the pass booking form on b-europe does not show full fare tickets.

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Perhaps your browser is storing cookies that flips it across to the standard booking, I don’t know.

Try using a different browser.

At this point it is too late, no passholder spaces are left for 31 Aug Paris (or Lille) to London in 2nd class.

There is one available train left on the morning of 1 Sep.