Hello PabloAlejandro
I am slightly puzzled because in the title you mention Swiss Pass (while I think you actually meant Swiss Travel Pass; in the text you asked about the Friends Day Pass for Youth.
Either way, the answer is the same: the Swiss Travel Pass as well as the Friends Day Pass are not valid for travel from Milan.
With either ticket you need an extra ticket for the stretch Milan to Domodossola, which is the commercial border station (Trenitalia / SBB). From there, the Swiss Travel Pass as well as the Friends Day Pass are valid. When you book the extra ticket, it comes automatically with the mandatory reservation to Domodossola, that’s all you need. In Switzerland, a seat reservation is not required, simply stay on board in Domodossola and show your pass to the SBB conductor once the train enters the Swiss rail network.
Regarding the routing: there is no need to travel to Geneva in order to get to Interlaken. This would rather mean a huge detour. Simply get off in Brig and change trains there in direction Spiez, from Spiez further to Interlaken. Or chosse the EC service from Milan to Basel SBB and change at Spiez.
You can find all schedules on www.sbb.ch/en. Tickets for Milan to Domodossola can be acquired easily from Trenitalia.
Book soon, the Eurocity trains are quickly sold out due to the festive season; alternatively you could travel on regional services.
Kind regards,
Roland