Try this link

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

If you have a mobile pass you will need to generate a Pass Cover Number

Eurail: https://www.eurail.com/en/book-reservations#/generatePassCoverNumber

Interrail: https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber

Their booking page for domestic trains is here:

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

Booking fee is €4 per order (which can be a return for multiple people).

You can also call SNCF. Press #85 for English. No booking fees, reservations are sent via email.

Thank you for your quick reply!

Unfortunately it doesn’t work. The travel.b-europe.com service page doesn’t show the train connection either (SNCF TER 3307). 

Do you have another idea?

Thank you!

For what route is this TER? Most of them are reservation-free and even cannot be booked.

TER Nomad trains can indeed not be reserved through online channels as far as I know. Try SNCF on the phone or otherwise it'll have to be at a French station. 

I looked it up on sncf.com, it's a Nomad one (Paris-Cherbourg). These are indeed the exception that do require reservations.

Indeed. I'd be curious to know if you can book via SNCF by phone. At the station, you can also book at the SNCF ticket machines (the white ones). Should be possible anywhere in France. The reservation costs €1.50.