Click the blue GET button to popup the download macOS High Sierra prompt dialog, it will ask you whether to download it now or later, click the Download button to start the download.And it will show the macOS High Sierra download page.
Click it will open the App Store in your current macOS ( my current os is Mojave ). So I need to download the macOS High Sierra. At first, I download the newest macOS Mojave follow the article How To Download macOS Mojave Installer And Convert To ISO File, and when I have done creating the iso file, I find VirtualBox does not support macOS Mojave, the highest version that Virtualbox supports is macOS High Sierra.