Adapted from https://www.howtogeek.com/289594/how-to-install-macos-sierra-in-virtualbox-on-windows-10/

hdiutil create \
  -o /tmp/Mojave.cdr \
  -size 2g \
  -layout SPUD \
  -fs HFS+J

hdiutil attach \
  /tmp/Mojave.cdr.dmg \
  -noverify \
  -nobrowse \
  -mountpoint \
  /Volumes/install_build

asr restore \
  -source /Applications/Install\ macOS\ Mojave.app/Contents/SharedSupport/BaseSystem.dmg \
  -target /Volumes/install_build/ \
  -noprompt \
  -noverify \
  -erase

hdiutil detach \
  /Volumes/macOS\ Base\ System/

hdiutil convert \
  /tmp/Mojave.cdr.dmg \
  -format UDTO \
  -o /tmp/Mojave.iso

Install dmidecode for MacOS from http://cavaliercoder.com/blog/dmidecode-for-apple-osx.html by running:

brew install cavaliercoder/dmidecode/dmidecode

Then run dmidecode -t system and dmidecode -t baseboard to get values for DmiSystemVersion and DmiBoardProduct below. Then customise the VM's CPU identity sets and DMI data.

VBoxManage modifyvm "MacOS Mojave" \
  --cpuidset 00000001 000306a9 04100800 7fbae3ff bfebfbff

VBoxManage setextradata "MacOS Mojave" \
  "VBoxInternal/Devices/efi/0/Config/DmiSystemProduct" "MacBookPro14,3"

VBoxManage setextradata "MacOS Mojave" \
  "VBoxInternal/Devices/efi/0/Config/DmiSystemVersion" "1.0"

VBoxManage setextradata "MacOS Mojave" \
  "VBoxInternal/Devices/efi/0/Config/DmiBoardProduct" "Mac-551B86E5744E2388"

VBoxManage setextradata "MacOS Mojave" \
  "VBoxInternal/Devices/smc/0/Config/DeviceKey" "ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc"

VBoxManage setextradata "MacOS Mojave" \
  "VBoxInternal/Devices/smc/0/Config/GetKeyFromRealSMC" 1