For existing customers: Once you are logged into your account, click on your name in the top right corner and select "Factor Bank". You'll then see a field labeled "Redeem A Gift Card". Paste the code there and click redeem.
For new customers: Your gift card code will be entered on the “Payment” step of signup. Beneath the entry form to enter a credit card, you will see a link that says “Invite code or gift card?”. Click the link, enter your code, and press “Submit”!