Once installation is complete, launch Revit 2019. You will be prompted to activate the product.
If you are a student or educator, you can get a free institutional license through the Autodesk Education Community, which provides a legitimate serial number and activation. Token Flex:
A Revit 2019 activation key is a unique code that unlocks the full features of Autodesk Revit 2019. The activation key is a 25-character code that is used to validate the software and grant access to its advanced features. Without a valid activation key, users can only use the software in trial mode, which has limited functionality and expires after a certain period.
A "full activation key" as promoted on many forums does not exist. What’s often offered are: