This guide is for UEFI-GPT Windows installs. Microsoft's bcdboot
utility can recreate the bootloader on any selected partition quite easily, and it won't break any existing EFI entries (unlike the bootrec /fixmbr on MBR installs).
This process can be done on a working Windows install, or on the Windows Installer. To access the Command Promt on the Windows Installer, press ShiftF10 after the Windows Setup window appears, or click Next > Repair your computer.
You can do this with Disk Managment as well, if you can boot the installed Windows.
Enter diskpart