Hyper-V won't boot customized Ubuntu Server ISO

I'm trying to create a customized Ubuntu Server ISO (using the instructions here) for use in Hyper-V VMs. I want to modify boot/grub/grub.cfg to add autoinstall to the kernel command line parameters to enable automated server installs.

menuentry "Install Ubuntu Server" {
    set gfxpayload=keep
    linux   /casper/vmlinuz   autoinstall quiet  ---
    initrd  /casper/initrd
}

If I create a new VM in Hyper-V and have it boot from an unmodified Ubuntu Server ISO and then manually modify the boot options in grub the auto-install runs flawlessly. However Hyper-V never boots from my customized ISO, skipping straight to PXE boot. The customized ISO does boot in KVM, so I must be missing something that just Hyper-V requires.

The specific command I run to create the new ISO after modifying grub.cfg comes from the guide linked above on customizing the install CD:

mkisofs -r -V "Custom Ubuntu Install CD" \
        -cache-inodes \
        -J -l -b isolinux/isolinux.bin \
        -c isolinux/boot.cat -no-emul-boot \
        -boot-load-size 4 -boot-info-table \
        -o custom.iso /path/to/iso/root

The base Ubuntu Server ISO I'm modifying is 20.04.3. My host is Windows 10.

Is there a way to modify the mkisofs command to include whatever Hyper-V requires, or a way to copy the exact settings from the standard ISO, since that works with Hyper-V already?

I figured it out for Ubuntu 20.04. You will need to create the ISO with a boot image. This is found on the iso it self it's called efi.img.

Sadly this does not work anymore with Ubuntu 22.04/10 as the efi.img is no longer present on the iso. I have yet to find a way to create the custom iso so it will boot in hyper v