Microsoft Windows Insider program allows consumers and professionals to preview the upcoming features of Windows 10 before they are shipped to consumers. To join the program and install the preview builds, you normally need a Microsoft account and a PC running Windows 10. An independent developer has now created a command-line script called ‘Offline Insider Enroll’ that allows Windows 10 PCs to enroll in the Insider program without the need to link your Microsoft account to download and install any Insider preview builds. The developer explained that this script takes advantage of ‘TestFlags’ registry value to enroll a user into the program.
Source: https://www.bleepingcomputer.com/news/microsoft/how-to-join-the-windows-insider-program-with-a-local-account/