You dont need to embed that, it can be read from the filesystem. You only have to be sure that your hd can be mounted without extra drivers - thats default for ubuntu kernels when you dont use raid. But in the case you want to boot with initrd, thats a piece of cake as well.
As you see you have to write the path to the initrd relatively to the efi partition, you can add a \ in front, but that is optional. You have to use \ and not / because that's the UEFI syntax. So whats your problem now? Btw you can use the root=UUID=... statement you usually get by
sudo cp /boot/initrd.img-$(uname -r) /boot/efi/EFI/ubuntu/initrd.img
sudo efibootmgr -c -d /dev/sda -p 1 -l '\EFI\ubuntu\linux.efi' -L 'Ubuntu direct with initrd' -u 'quiet root=/dev/sda2 initrd=EFI\ubuntu\initrd.img'
if you want when you use an initrd as well, thats the uuid of the filesystem - this would even work when you use mbr partitions - the efi fat partition must be a PRIMARY one in that case!
echo root=UUID=$(blkid -p -s UUID -o value /dev/root)