Add Windows 10 to GRUB OS list

I recently got a computer from school. It came with Windows 8.1 installed. I then installed another copy of Windows 8.1 (so it would use the built-in Windows Pro license) and upgraded it to Windows 10. I then installed Ubuntu alongside both Windows 8 and 10.

Note that I made a separate 255MB ext2 partition for /boot.

Now, when I boot the computer, I first see a GRUB prompt (I already un-hid the grub prompt 'cuz I like it that way) with Windows 8 and Ubuntu options. However, there is no Windows 10 option. If I select Ubuntu, the computer boots into Ubuntu. However, if I select the Windows 8 option, I then get sent to the Windows bootloader, where it then gives me options to boot either Windows 8 or Windows 10. (However, by this point, the computer has already loaded the kernel and all. Apparently Microsoft misunderstood the concept of a bootloader and decided that that meant "boot practically the entire system before actually showing the bootloader". Typical Microsoft.)

How can I disable the Windows bootloader and add a Windows 10 option to GRUB?

EDIT: Someone has answered telling me to disable the Windows bootloader. However, I still do not know how to add Windows to the GRUB OS list. Can anyone help with this?

EDIT 2: After removing Windows 10 from the Windows bootloader with EasyBCD, it seemed that, after a few reboots, it would continually unhide itself and Windows 10 would reappear. However, I then later deleted Windows 10 from the list of OSes in msconfig, and it seems to have stayed deleted.

However, I STILL have not managed to get Windows 10 to appear in GRUB. It's as if GRUB simply doesn't detect it.

Also, I'll put this in the question so that people see it - I have tried using boot-repair, however, it has NOT helped any.

Sometimes os-prober is wrong.

Look at the UUID of your Windows 10 partition, for exemple:

sudo blkid /dev/sda2

Then edit /etc/grub.d/40_custom, and at the end of the file add:

menuentry "Windows 10" --class windows --class os {
   insmod ntfs
   search --no-floppy --set=root --fs-uuid $your_uuid_here$
   ntldr /bootmgr
}

Don't forget to change the UUID.

And finally update your grub config file:

sudo update-grub

Source

This solution provides a proper entry in the grub2 menu and chainloads directly into Windows 10 without reference to the BIOS. It comes from this webpage so I claim no credit. It was such a relief to find a working solution.

Briefly, edit /etc/grub.d/40_custom and add:

menuentry 'Windows 10' {
    search --fs-uuid --no-floppy --set=root CC66-4B02
    chainloader (${root})/EFI/Microsoft/Boot/bootmgfw.efi
}

To find the UUID for the --set=root line (CC66-4B02 in the example) you use sudo fdisk -l to identify the EFI partition then sudo blkid /dev/sda1 (or whatever) to find the UUID of the EFI partition. Note it's not the Windows partition but the EFI one you need. Once you've saved the edit, run sudo update-grub to generate the /boot/grub/menu.cfg file and then restart to test.

Use:

sudo update-grub
sudo grub-install /dev/[BOOT PARTITION eg. Sda6]

commands in terminal of Ubuntu, probably it will solve your problem.

By this command the system will automatically detect your Windows installations and add them to the GRUB list.