Goofocoqu76 Do you travel with a TGV from Paris to Bellegarde and with a regional train from Bellegarde to Geneva?
Your Swiss Pass is not valid from Bellegarde, who is still in France. I think that following rules apply:
If you take a train who stops at every station between Bellegarde and Geneva, you could buy a ticket from Bellegarde to La Plaine for France and use your pass from La Plaine to Geneva for Switzerland.
If the train doesn’t stop between Bellegarde to Geneva, you have to buy a ticket from Bellegarde to Geneva.