There are no free trial subscriptions with opt-in "continue paying" as far as I know. I mean, why would Apple and Google build that? They make more money with trial + automatic billing.

I once built such a trial option myself. You could start a free trial (just within the app, no "native trial" involved) and then sign up for the paid plan with native in-app purchases once it ran out.

It got rejected by Apple. They insisted I use the native app store trial which is opt-out.

Of course they support an opt-in model, they can't really choose to not support it. The "free trial" can just be the app giving you all the features for a week, and then the app can prompt you to sign up (opt-in) for a subscription with Apple/Google when that ends. No CC number until after the trial ends.

EDIT: I'm wrong, comment next to mine says Apple has rejected this. Maybe Google wouldn't though?