You will now have a file called HighSierra.iso on your Desktop. This is a legitimate, unmodified, and bootable macOS High Sierra 10.13.6 installation disk image.
There is no direct official download link from Apple for a standalone macOS High Sierra.iso . You must create it from an official installer. Apple remains the only legal and secure source for the macOS installer. Macos High Sierra 10.13.6.iso
After closing VirtualBox, you must run specific command prompts in your Windows command prompt to make VirtualBox mimic real Apple hardware. You will now have a file called HighSierra