simply add uefi entry menu by booting into your qubes os installation medium, rescue qubes os, wait and type 3 to skip shell, lsblk to examine your external ssd, if it’s /dev/sdc then run
efibootmgr -v -c -u -L "TheEnemy" -l /EFI/qubes/grubx64.efi -d /dev/sdc -p 1
-p 1 is stand for partition 1, as default in most os, partition 1 is your efi partition.