Hi

I have an interrail ticket and I want to take a tgv train from paris to basel. I see that I have to get a seat reservation, which I know how to get. But I also see that there is a supplement required and I really don’t find any specific information how to get this supplement. Is it already included if I buy the seat reservation? If not, where can I get it and how much will this be?

I really hope someone can help me with this!! :) as I am looking for this info already for hours.

Best of wishes from Switzerland
Rahel