Do I need to opt in on every application?

No. The whole point of the subscription is that you opt in once and Priority Review applies to every qualifying job you apply to from then on. The system stamps "priority" on each new application automatically.

You don't have to click anything special, fill out any extra form, or pay anything per-application. Just apply normally — the priority flag is added behind the scenes.

You'll see the priority status reflected in your /applicant/applications list (each row is badged) and on the recruiter side they'll see the gold "Subscriber" pill next to your name.