To redeem a turbotax service code online, visit turbotax website, sign in to your account, and start a new return or open an existing one. Before checkout, look for the “Got a service code?” link on the payment screen. Click it and enter your 16-character code. The discount or offer will apply automatically. Ensure the code matches your product version. If the code doesn’t work, double-check for typos or contact TurboTax support for help.