Free Swiss Pass fares

I’m not sure what you mean by “sign up online for a train”. You just get on the train you want and show your pass to the conductor when they ask for it.

are you trying to go somewhere the pass doesn't cover at full price?

Maybe clarify a little what exactly you are trying to accomplish? If we understand we can help..

You do not "sign up" for a train. Not on line, not off line. I think you may be a bit confused as to how trains work.

You indicate that you will have a Swiss Travel Pass. So you are not in Switzerland yet. Hence there is nothing you need to do.
Once in Switzerland you go to the SBB Travel Centre at the Airport, and purchase the Swiss Travel Pass. Or purchase it in advance.

Anyway, with the pass all you need to do to take a train is find it, board it, take a seat, and show your pass once the conductor comes.

I'd feel better if we knew what train(s) you're planning to take. I believe the Swiss Pass doesn't cover the full fair on some high-mountain railways.

The Swiss Travel Pass does not cover every train/gondola journey. Some are only discounted. This is a great site: https://www.myswissalps.com/swisstravelpass/validity

Perhaps you are trying to make a reservation for one of those journeys that is not fully covered?

The majority of trains do not require (or even allow) reservations. If you have a pass, you just board the train. When asked for your ticket, just show your pass.