core/system/boot/grub2/files/42_custom

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry 'Windows 10' --class windows --class os {
set root='hd0,gpt2'
chainloader /EFI/Microsoft/Boot/bootmgfw.efi
boot
}