The best way to check is to give it a try. I'm not sure is it still possible to redeem codes, as I don't see it on my Billing page, but check your Billing page to see is there option to redeem the code.

Note that if this still exists, you can redeem only one promotional code per your account and you need to add credit card or pay the minimal amount with PayPal, in order to confirm your account.

You can also check out with the Support team about this or other promotional codes.

If you want to earn free credits, make sure to check out the DigitalOcean Referral Program!