You still need a valid, paid product key to activate Windows 8.1 after installation. Using ei.cfg merely allows you to choose which edition to install from a multi-edition ISO.
Its primary purpose is to tell the Windows installer which edition of the operating system should be installed and what type of licensing mechanism should be used. It acts as a gatekeeper that determines the flow of the installation wizard. ei.cfg windows 8.1
Here’s an informative review of in the context of Windows 8.1 — what it is, how it works, and why you might want to use or remove it. You still need a valid, paid product key
