First, make sure you have secure boot disabled from the firmware settings. Once you are in the grub command line, type ls
to list all partitions
grub>ls
(hd0) (hd0,gpt1) (hd1) (hd1,gpt8) (cd0))
Type ls (cd0)
to get the UUID of the device.
grub>ls (hd0,gpt1)
Partition hd0,gpt1: Filesystem type fat - Label `CES_X64FREV`, UUID 4099-DBD9 Partition start-512 Sectors...
Note the UUID of your USB drive, shown in the above command
Type the following commands
insmod part_gpt
insmod fat
insmod search_fs_uuid
insmod chain
search --fs-uuid --set=root 409-DBD9
Replace the UUID
of your device
Now we select the EFI file to boot from
chainloader /efi/boot/bootx64.efi
boot
That’s it; that should boot the USB drive.