How to book Goldenpass VIP seat with saver day pass?

Hello, I want to take 8th Oct GoldenPass Panoramic train (10:50 Montreux - 12:57 Zweisimmen).

I already have a saver pass (1st class), and I also find the Goldenpass VIP seat reservation link as below.

But it seems I can only buy a normal rate ticket on the site above, which means I double payed saver pass (1st class) fare and Goldenpass train fare.

So the question is:

In case that I already have a saver pass, where can I pay additional GoldenPass VIP seat fee? (looking for an online link)

Choose “Reservation only” from field “Reductions”.

Best regards,

Roland