So, if I understand you correctly, you are currently using the Windows bootloader? (meaning you probably installed Fedora before Windows)? Are you able to boot to both systems? If so, what are you hoping to accomplish by installing Grub?
If you want to proceed, I believe you should be able to install Grub from your current Fedora install. You can find information about Grub here.
NOTE: The commands given here may make your system unbootable, please use them at your own risk
Before doing anything, I would create and burn a bootable Windows iso using the media creation tool on this page from your Windows partition that you would be able to use to fix your Windows boot manager if anything goes wrong (download the tool and see "Using the tool to create installation media (USB flash drive, DVD or ISO file) to install Windows 10 on a different PC (click to show more or less information)" on the same page)
After that, I would boot into Fedora, open a terminal, and run the following command
grub2-install /dev/sda
I would then reboot and see if both Windows and Fedora are listed. In the case that Windows is not listed, I would go back into Fedora and run
grub2-mkconfig -o /boot/grub2/grub.cfg
Which I believe should create a new config file with a (hopefully) detected Windows entry that you should be able to load upon reboot.
If all fails, and you want to revert, I believe you should be able to boot the Windows image you created earlier, and use the Startup Repair tool to recover (and if that fails, there should be more you can do using the Windows image and selecting a command prompt, I believe this command should work, e.g., bootsect.exe /nt60 C: /mbr )
