Do I have to announce the Darius trigger so he gets the +2 or does that happen automatically?

Yes, it happens automatically — but you must acknowledge it or risk losing it.

Darius's ability has no "you may," so it's a mandatory trigger (FAQ #9603). When you play your second card, the trigger is placed on the chain automatically — you don't get a choice.

However, per Tournament Rule 506.3, you are the "accountable player" for your own triggers. You must acknowledge it by the time it would have an observable impact — for Darius, that's the +2 Might or the readying (Rule 506.3.d.3 and 506.3.d.6). If you take a new game action (e.g., moving to combat, playing another card) without acknowledging it, the trigger is considered forgotten and never resolves.

In practice: you don't need to pre-announce, but before you do anything else after playing your second card, say "Darius triggers" or clearly apply the buff and ready him.