If you already have a Seamwork membership and want to add a gift card to your account, just follow the instructions here.
If you're new to Seamwork, start by entering your gift code in the "Redeem it here:" box on this page: https://www.seamwork.com/join#gift
Next you'll create a Seamwork account. You'll be prompted to enter your payment information. A stored payment method is required in order to activate a membership, even when you're paying with a gift code.
Seamwork memberships automatically renew, so when your membership is up for renewal your payment method on file will be charged.
If you want to ensure your payment method isn't charged, there is a work-around. Go ahead and provide your credit card info when you redeem your gift code and then cancel your membership from your account page as soon as you complete the activation:
Your membership will remain active until the end of the current billing period (ie, this date next year) but you will not be charged again. You will be able to undo the cancellation request at any time before then from your account page if you decide you'd like your membership to renew at the end of the current billing period.