Using your newly created ISO:
ElCapitan_10.11.6.iso to the optical drive.VBoxManage setextradata "YourVMName" "VBoxInternal/Devices/efi/0/Config/DmiSystemProduct" "MacBookPro11,3"Open Terminal and paste:
hdiutil create -o /tmp/ElCapitan -size 8000m -volname ElCapitan -layout SPUD -fs HFS+J
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/ElCapitan --nointeraction
(This takes roughly 15-20 minutes.)
For users who need to download OS X El Capitan, either for installation on a compatible Mac or for creating a bootable USB drive, there are a few considerations:
If you download a 10.11.6 ISO from any third party, you must verify its SHA-1 hash before running it. Use 7-Zip (Windows) or shasum -a 1 filename.iso (Mac/Linux). Mac Os X El Capitan 10.11 6 Iso Download
A legitimate, untouched 10.11.6 Installer (converted to ISO) should have a hash roughly matching this (Note: Exact hash varies by conversion tool, but look for consistency):
Search for community-verified hashes on Reddit r/hackintosh or InsanelyMac forums. Using your newly created ISO:
Red Flags: