Follow these steps to redeem your Roblox Gift Card:

  1. Sign in to your Roblox account in your browser
  2. Go to the Game Card Redemption Page page
  3. Enter the Roblox Gift Card which you can find under My Withdrawals by clicking on "Show Code"
  4. The balance has now been added to your Roblox account

Note: You can find the official instructions here.