After doing some research and trying to figure this out, I found an article online that mentioned holding ESC while using the power button to turn on the computer. After further testing I figured out that I needed to have the Secure Boot option in the BIOS disabled or else I got a security error when I tried to boot to the USB device. I changed this by going to the Security menu and disabling the "Secure Boot Control" option.
I also had a setting called "Launch CSM" under the boot menu which I had to enable (did not enable "Launch PXE OpROM"). With the secure boot option disabled and launch CSM enabled, I was able to hold ESC while powering on the laptop and boot into my USB device. Doing this gave me a menu at boot up where I could choose what to boot into. I chose the option: "UEFI: General USB Flash Disk 1.0" (I had created Macrium Reflect rescue media and during the setup checked the box "Enable multiboot MBR/UEFI USB support", so I am not sure if you would still want to choose this otherwise).
I was working on an ASUS laptop (SonicMaster I think) using the Aptio Setup Utility - Copyright (C) 2012 American Megatrends, Inc. If you are using a different BIOS the steps for this may not be the same, but hopefully there is enough information here for you to make this work on your computer.
I also noticed that after getting this to work using the above method, I got some extra options in the BIOS that were previously not there. This included extra boot options (boot menu) and extra boot override options (save & exit) menu. The new boot override option "UEFI: General USB Flash Disk 1.0" worked great after the steps above.
Before I had found this option, I had also seen online where people had mentioned holding Shift while clicking the restart button under Windows 8 to get a menu option to boot to a device. [https://askleo.com/how_do_i_boot_from_cddvdusb_in_windows_8/]. This didn't work for me, but you may want to try this first to see if it works for you.