Well it's pretty clear from what you've written : 

Prices for this journey will start to be available 90 days before departure.

That's for interrail.eu/eurail.com

You could already buy it here but there's really no need to (+ there's a 4€ booking fee per order) :

https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

Simply wait a few days, don't worry seats won't fill out as fast as the Eurostar anyway

Thank you for your reply.  Traveling with a little5 year old and husband just want to be able to get seats.

If you're travelling with 2 other people, then it'll be cheaper on b-europe so go ahead if you want.

But don't worry seats won't sell out until a few days before

Enjoy your trip

Thank you.  Can you use the Eurail pass booking with B-Europre train?

I purchased a Eurail 4 day pass for 1 month. 

Yes of course, you only buy seat reservations there.

They'll ask you for a pass number, you should use that page :

https://community.eurail.com/mobile-pass-62/pass-cover-number-generator-is-live-5653

Thank you very much for your responses.  

I am not seeing where I can link my Eurail Pass to  B-Europe to purchase seat reservations.

What do you mean ? I can go all the way to this 

And then you enter the number created by the generator (link above)

Let me know

what site is this?  are you on an app?

It's https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish on my smartphone.

Btw reservations are completely separate from the Eurail app. They won't appear anywhere. When a ticket inspector comes, you simply show them both your pass and the seat reservation

I went to the site that you included from your last message. When I login to b-europe on my laptop I can see the train from London to Paris and Paris to Bordeaux but I cannot link my Eurail pass to the ticket when trying to pay. When I login to b-europe on my smartphone I do not see the train from London to Paris but do see the box to link our Eurail pass to each passenger.

How can I book the entire trip from London to Bordeaux via Paris and link my Eurail passes we already purchased?

You can't book the whole trip on the same b-europe web page. There is one page for the Eurostar and Thalys and another web page for domestic French trains.

Yes, but even if we look at the first leg of London to Paris only. We still do not see the option to link our Eurail pass when logged in on the laptop.

Please post a screenshot where it doesn't work.

When the website asks for a pass cover number, use this :

screenshot i took using my iphone.  It should not be this hard.  I can book it on Eurail website, but the train leaving London to Paris time that I need all seat reservations are sold out.

You're not on the right page in b-europe.com

It should not cost 637€ !!! This is for regular tickets.

Here is the correct link : https://www.b-europe.com/EN/Booking/Pass#TravelWish

But if the train is sold out on eurail.com it'll be sold out there too. Simply take an earlier or later train, there are multiple trains per day to Paris and then to Bordeaux. 

Which was the specific connection you wanted ?

Please delete the screenshot too, it shows some private info

EDIT There are no seats left on the 09:31 Eurostar so you'll need to take the 10:26 Eurostar, not a big problem. 3 x 30€ = 90€

Consider at least 1h-1h30 margin to cross Paris

The TGV at 15:08 from Paris-Montparnasse should fit perfectly with the 10:26 Eurostar.

Around 9am-9:30am in London and at 5:15pm in Bordeaux (incl. the 1h time difference). Isn't that incredible ? :)

OMG! thank you so much for your guidance.  I was able to booked our London to Paris train with seat reservations.

You're welcome ! I'm glad to know it worked :)

Can someone please provide a link to book Eurail train from Venice to Rome using B-Europe?

Venice to Rome (and other Italian day trains) is best booked on tickets.oebb.at. Do NOT use "Seat reservations only” but add the Interrail/Eurail discount for each person and then select "One-way tickets and day tickets”.

Trains in Italy (and many other countries) are best bought on tickets.oebb.at (no booking fee).

Add Eurail/Interrail as a discount and select one-way tickets.

Let us know if it doesn't work. Keep in mind that the Italian summer timetable hasn't been entirely published yet.