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.