Depends on your choosen connection (which you didn't tell).

If you'd like to go by spanish AVE, you need to do your reservation for this one at a spanish train station. If you'd like to go at 8.19 you'd need a reservation just for this one when connecting to a TER to Genf.

If you'd like to go by french TGV, you may do your reservation with https://www.raileurope.com/pt (desktop version only; add you railpass bottom left before searching). 

For trains in Switzerland itself, a seat reservation is not necessary. 

BTW: August 1st is national holiday day in Switzerland. There will be many events and fireworks; you may have a look for these.

First of all, thank you very much for the tips/information! I really didn't know about the national holiday in Switzerland!!

So I won't be able to buy my seat in advance? This would be the same one you mentioned at 8:19 in the morning. 

My connection would be the one in the attached/below image!

Thanks again!! 

Yes, that train is not possible to book online, only at Spanish ticket offices.

The 09.28 TGV can be booked as detailed above by @Hektor .