Pogacar123
You can buy any online tickets with HF PLUS as payment method which are offered/available in the SBB Mobile app or the sbb.ch ticket webshop, provided the tickets are for yourself and not for someone else. Those cable car tickets not sold online (on-site only) are usually with smaller, privately owned cable cars who are not cooperating with the Swiss public transport system. This doesn’t apply for the popular examples you mentioned.
Note that you benefit from that 40% discount (HTP 1000 for youth) only if fully using the 1000.- credit, after you’ve paid the 600.-
However the standard Half Fare Travelcard (1 year for residents, 190.-) and the Swiss Half Fare Card (1 month for foreign visitors, 120.-) do offer an even better 50% discount in the first place. Therefore those half fare products would usually be purchased as a first step (if the sum of purchased tickets would exceed 380.- or 240.- at full fare) before getting a PLUS package as a second step.
If assuming you are a visitor staying less than a month and your travel plans would incur a total of 700.- full fare tickets, paying those with a HTP 1000 Youth would cost you 600.-, while with the Swiss Half Fare Card it would cost 120.- + 350.- = 470.-
EDIT (I got interrupted while typing the stuff above):
As you already have a half-fare travelcard, the HTP1000 youth obviously starts making sense as soon as the total of purchased tickets within a year exceeds 600.- @ half fare prices.