Hello Koollueting25
Holding a Swiss Travel Pass, you are entitled to purchase class upgrades at half fare (½). You can choose between the point-tp-point class upgrade, which is only valid for a specific route, or a day pass upgrade, with it you can travel in 1st class for 1 day in all trains.
You can purchase it following the links mentioned on the website which URL you have pasted in your question.
Does that answer your question?
My advice, first book the seat in 1st class, then once the seat is saved, book your upgrade. A 1st class ticket is not needed to make a seat reservation in 1st class.
Kind regards,
Roland