Setting up a Microsoft Surface device to boot from any other type of media other than the internal drive can be tricky.

Below are the steps to access the device's BIOS, make the necessary changes to they system, and to boot from your target device.

The first requirement to booting from a USB drive is to change the Surface device's boot order:
1) Make sure the Tablet is powered off to begin.
2) Press and hold down the volume up (plus) button found on the side of the device.
3) Press and release the power button while still holding down the volume up button.
4) The device should boot and it should take you to the BIOS of the Tablet.
5) Select the 'Boot Configuration' tab. This should display a list of all possible boot devices.
6) From the list, select the USB option and move it to the top of the list.
7) Next, go to the 'Security' tab.
8) Make sure the 'Secure' option is set to 'Microsoft and 3rd Party devices'.
9) After setting this, turn off the tablet. All changes should be saved automatically.

Booting from a USB device:
1) Plug in a bootable USB containing boot media, along with a mouse, and the USB Ethernet
Adapter. (This usually requires a USB hub as most Microsoft Surface devices only have a single USB port) 
2) Power on the tablet.
3) The USB boot device should boot up automatically, as it was selected as the boot priority in the steps above.

     Note: If you are given any issues here, try having the USB drive as the only USB device attached to the tablet.