The bookings open usually 6 months in advance, for TGV Lyria.

Note you can save quite a bit of money by taking an ordinary domestic TGV to Mulhouse, and take local trains to Switzerland /Zürich. These domestic TGV trains are not open for booking yet (usually 3-4 months in advance). TGV Lyria has extortionate seat reservations.

Alternative ways on how to book these, are found in the link below:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index3.html?postid=37042#post37042

Thank you Brendan.  I’m happy to pay the reservation, I just can’t put it in the cart?  I can see the bookings on SNCF and make them there but I cant put in our Eurail pass numbers?

Thank you Brendan.  I’m happy to pay the reservation, I just can’t put it in the cart?  I can see the bookings on SNCF and make them there but I cant put in our Eurail pass numbers?

Please visit the link that BrendanDB posted, there it is listed where you can buy the reservation. If you have further questions, we are happy to help :)

Thank you

So on the Eurail one I can see it, but there is no option to add it to the cart and buy it.
 

On SNCF I see this but cant book it either or put in my Eurail number

Am I just too early in booking maybe please?

 

Like I said before there is all the information you need on the link, and here you see, you can’t book via SNCF Online. 

Booking reservations for TGV Lyria trains is possible via:

• Interrail.eu/Eurail.com (2€ booking fee per person and train) (You need to have a Pass to book)
• French railways ticket office or by phone (press #85 for English, no booking fee, delivery via e-mail).
• travel.b-europe website for national rides in France (4€ booking fee per booking, Pass Cover Number needed, request it here) 
• SNCB/NMBS ticket office or by phone
• SNCF Ticket Machines for rides in France
• Swiss ticket office or by phone (delivery by postal mail).
• NS International ticket office or by phone.