Have you already downloaded the installation file? If not, the FAQ below has the steps to get the file.
When you run the downloaded installation file, at some point near the end of the installation process it will ask you to activate the software by logging into your Online Intuit Account (or creating one if you don't have one), and a screen will ask you to enter the license code. See this FAQ and images:
FAQ: How do I get started with desktop TurboTax?
https://ttlc.intuit.com/turbotax-support/en-us/help-article/license-information/install-turbotax-cd-...