If you need the premium features for a specific task (e.g., online banking security for 3 months), a 90-day trial via a legitimate giveaway is fantastic. Otherwise, stick to the free tier.
If you have found a legitimate activation code, here is how to apply it:
Searching for on sketchy forums exposes you to:
While not a single "key," this gives you 60 days (or 90 with a third email cycle). For a truly seamless experience, consider purchasing a 1-year key for ~$20 during Black Friday sales.