You can buy the ticket from Spiez to MIlan ( the direct train, no changes) online from either Trenitalia or SBB. Either one will give you an opportunity to enter your Swiss Pass and it will deduct the cost of the travel covered by the Pass. Seat reservation is included.
For the 14:05 direct train from Spiez to MIlan, the Swiss Pass reduces the cost of your ticket from €39 to €24. On SBB, the price with the Pass included ed is 28 CHF. So the Trenitalia price is slightly lower (24€ = 26.7 CHF at today's rate). But the SBB site allows you to choose your seat, at least whether you want a window or aisle seat. I don't know if you can choose on Trenitalia.
To find the option for entry of the Swiss Pass on Trenitalia, choose that direct train and then scroll down below the price chart to "see other offers". Choose "GA Switzerland". (GA stands for General Abonnement, or price reduction). If on SBB, where it asks about passes for Switzerland, choose "GA 2Kl ( assuming younare buying 2d class), even though it only mentions the full-year Pass used by Swiss residents. Your Swiss Pass has the same effect.