u-boot syntax question

What's the correct device syntax for u-boot's usbboot command?
Here's a session transcript:

U-Boot> usb storage
  Device 0: Vendor: External Rev: 0204 Prod: USB3.0          
            Type: Hard Disk
            Capacity: 953869.7 MB = 931.5 GB (1953525168 x 512)
U-Boot> usbboot
** No device specified **
U-Boot> run usbboot
## Error: "usbboot" not defined
U-Boot> usbboot 0:1
** No device specified **
U-Boot> usbboot disk0:1
** No device specified **
U-Boot> 

I've tried 
starting USB...
Bus usb@7e980000: USB DWC2
scanning bus usb@7e980000 for devices... 6 USB Device(s) found
       scanning usb for storage devices... 1 Storage Device(s) found
Hit any key to stop autoboot:  0 
U-Boot> usbboot disk0p1
** No device specified **
U-Boot> 

What's the correct syntax for specifying a usb boot device?
Or, am I doing something else wrong?

Thanks for reading,

bob prohaska