When installing SQL Server 2019 Reporting Service or 2017 and later, you will be ask to enter product key.
According to Microsoft article, product key can be retrieve from SQL Server 2019 installer.
Actually there is another way to get it.
When we run setup.exe from the media, the Product Key is picked from DefaultSetup.ini file located under RootX86 or Rootx64 folder. This file is also used to make slipstream media (refer KB)