No option to boot Windows in Startup Manager
Here are the steps I would use to install Windows 10 your Mac. I assume you have a Windows 10 iso file.


Install the current macOS operation system to your Mac. Currently this High Sierra version 10.13.2. The accomplishes two tasks. First, you will get all the logic board firmware updates. This is done in the background, so you will never know if a firmware update occurs. You used to have to manually install firmware updates, but as you can see from the legacy firmware update site, Apple now does update automatically. Second, you will get the latest version of the Boot Camp Assistant application. You can use this application to create the USB Windows 10 installer flash drive.


  Note: The newer Macs can install Windows 10 without the use of a USB flash drive.

Use the Boot Camp Assistant to install Windows 10. 

If you do not want macOS installed on your Mac, then you can delete macOS during the installation of Windows 10. If you reach the window shown below, select "Custom: Install Windows only (advanced)". 



Next, remove macOS by deleting all the partitions. The result should be the same as shown below.



With the unallocated space highlighted, select "Next". The windows installer will automatically create the correct partitions.


The answer to your question

You Mac will not boot from the flash drive because the drive is not FAT formatted using a MBR partitioning scheme. If your drive is FAT formatted using a MBR partitioning scheme, then the firmware on your logic board is not up-to-date.