Boot Win10 from an ISO-Image on a certain partition using GRUB2

I have bought a laptop (Lenovo ThinkPad T520) with a locked BIOS. So I can't change the boot order of the laptop. The upmost device in the boot order is the internal HDD. Fortunately there is a working Linux with Grub2 installed, which I have access to.

I also want to install Windows 10 now. How do I have to change the Grub2-configuration in order to be able to boot the Windows-installation from a Win10-Boot-Medium (ISO)?

If you're using UEFI and only looking for a temporary solution (to run the installer), extract the Windows installer contents to a data partition (so it doesn't have a system on it). Make sure to have it at the top level, not in a folder, otherwise the installer fails to load. Then in GRUB enter the command line (c) and find the hard drive using ls, then enter chainloader (hdX,X)/efi/bootx64.efi and boot. This should boot into the Windows installer. After it finishes installing, you can delete the installer's files and folders:

autorun.inf boot bootmgr bootmgr.efi efi setup.exe sources support