Hello Krist
You can reserve seats on eurail.com, with a fee of 2 €, I don’t know if it works if you don’t buy the pass on the same site.
You can also reserve seats on raileurope.com, currently without any fee. You must give your pass number, but it is not necessary that the pass is already activated. On this site you can also choose seats with a surcharge of 2 €.
Refunds are also possible online according the conditions of Trenitalia (with a fee and only tickets above 10 €). Eurail doesn’t refunds the 2 € fee.
You can import the reservations in the Trenitalia app to make exchanges. After an exchange, the tickets are no more refundable,
To be honest, I always reserved my seats from and to Domodossola, never from Spiez, because I have a GA.