Start Parallels Toolbox for Windows. If you haven't signed it yet, you will see the registration window. You should either sign in to your existing account or create a new one.
After you sign in or create an account, you will see the Try or Buy window, where you can also switch to the activation window. Click on I Have a Key to enter your license.
Enter your activation key for Parallels Toolbox, and click Activate.
Your copy of Parallels Toolbox for Windows will be activated.