If you previously owned Office 2016, you can often find your key through your Microsoft Account under "Services & Subscriptions".
Many reputable third-party retailers offer discounted product keys for older versions of Office, allowing for a legal and safe activation without the high cost of a full current subscription.
Many "activators" are actually trojans designed to steal your personal data or lock your files for ransom.
If you have a valid product key, follow the official setup process: Go to the official Office Setup page. Sign in with your Microsoft account. Enter your 25-character product key.
Cracked software cannot receive official security patches from Microsoft, leaving your computer vulnerable to new exploits. Legitimate Ways to Activate Office 2016
If Office was already installed on your device, you might be able to retrieve the key by running the command wmic path softwarelicensingservice get OA3xOriginalProductKey in an administrator Command Prompt.