The SwissPass is a product for Swiss residents; it’s a red plastic card on which the tickets can be stored. The Swiss Travel Pass is for tourists, you do not have the plastic card. As a consequence, you cannot buy reduced tickets online with your Swiss Travel Pass. This is only possible in Zermatt with the ticket clerk at the cable way base station.
See the website of Matterhorn Paradise, in the FAQ section, it explains what I just said above:
You can buy the reduced tickets online if you have a SwissPass with a valid half-fare/GA travel card. The ticket will be charged directly to your card. Please remember to carry the SwissPass card with you (mobile SwissPass is not possible). Otherwise, we ask you to buy your reduced tickets at one of our sales points upon presentation of your reduction card.
Does that help?