First make sure you have secure boot disabled from the firmware settings. Once you are in grub command line type ls to list all partitions

grub>ls 
(hd0) (hd0,gpt1) (hd1) (hd1,gpt8) (cd0))

Type ls (cd0) to get UUID of 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 you usb drive, shown in 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 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