I have yet to see this option too. The last comment on this forum I saw from an official SquareSpace rep is that they didn't support this. I know Stripe itself does support this, and I'm even considering using Stripe more directly then relying on the SS Subscription feature.
This is frustrating because I have a "One Week Trial" product, a Monthly subscription product, and there is no way to link to the two. I know there are some weird rules about reminding customers and such but Stripe has all these features built-in already.