To install Windows 10 on Ubuntu, you will need a full primary NTFS partition created. This can also be done through the Ubuntu 16.04 install method.
Windows Installation will be performed on a bootable DVD/USB stick… the second step is to install Windows 10.
Get Grub working on your Ubuntu computer.
How do you install Windows when Linux is already installed?
More Information
Remove native, swap, and boot partitions used by Linux: Start your computer with the Linux setup floppy disk, type fdisk at the command prompt, and then press ENTER. ...
Install Windows. Follow the installation instructions for the Windows operating system you want to install on your computer.
How do I install Windows Vista on Linux?
How Do I Install Vista On Ubuntu?
If possible, the USB stick must be at least two gigabytes in size.
Verify that the BIOS is set up correctly and that the computer has taken to USB.
It is extremely easy to burn pictures to your USB stick with a program.
Keep the USB stick plugged in and reboot the Windows Vista.
Dual-booting Vista and Linux / releases of Ubuntu
First, start off with a machine with Vista already installed on it and all data securely backed up
(use Vista's Backup and Restore Center or use a similar product).
Next, download a Linux distro. ...
CREATE USB or Burn a CD of the ISO you downloaded and put it on one side for a moment.
Fire up Windows Vista.
To start the installation, open up a terminal window by pressing Ctrl + Alt + T
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
Start Windows Installation from bootable DVD/USB stick
https://askubuntu.com/questions/6317/how-can-i-install-windows-after-ive-installed-ubuntu
https://silentinfotech.com/blog/steps-to-install-windows-10-on-existing-ubuntu-16-04/
Step 1: Prepare partition for Windows Installation in Ubuntu 16.04
To install Windows 10, it is mandatory to have Primary NTFS partition created on Ubuntu for Windows.
Create Primary NTFS partition for Windows installation using gParted OR Disk Utility command line tools. If you have existing logical/extended partitions in which you want to install windows, then you need to delete that partition and create new Primary Partition. (NOTE: All data in the existing logical/extended partition will be erased. Because you want Windows there.)
https://ubuntu.com/search?q=INSTALL+WINDOWS
With a bootable Ubuntu USB stick, you can:
Install or upgrade Ubuntu
Test out the Ubuntu desktop experience without touching your PC configuration
Boot into Ubuntu on a borrowed machine or from an internet cafe
Use tools installed by default on the USB stick to repair or fix a broken configuration
Creating a bootable Ubuntu USB stick from Microsoft Windows is very simple and we’re going to cover the process in the next few steps.
Alternatively, we also have tutorials to help you create a bootable USB stick from both Ubuntu and Apple macOS.
https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview
Installing Windows 10 without USB using Ubuntu, Step by Step
This step by step has been created from information provided by the OP, @Marcel Alexandru.
Backup the Target drive.
Create a 6GB NTFS partition on the hard drive and extract the Windows ISO to it.
Create a 20GB, or larger, NTFS partition on the hard drive for the Windows Installation.
Open Disks, (Gnome-Disks), and note Device, (/dev/sdx), and UUID of the Windows ISO extract partition.
For msdos partition table, copy the menuentry given in this question to /etc/grub.d/40-custom/*
Add ntldr /bootmgr as a line below chainloader +1
Edit menuentry, changing sda4 to sdax, msdos4 to msdosx (4 places), and 592C85254E2CD0B7 to UUID, (3 places), to suit step 4 above.
Run sudo update grub confirm that ntldr /bootmgr appears in grub.cfg.
Boot the computer into the newly created menuentry and install Windows into it's new partition.
Reinstall Ubuntu if desired, the GRUB bootloader will have been replaced with the Windows bootloader.
*Note from the OP: You don't have to reinstall Ubuntu to fix the grub loader. You can boot in the Ubuntu Live USB, open a terminal and open Boot Repair using the boot-repair command. If it's not available you can install it using the following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair sudo apt-get update sudo apt-get install -y boot-repair && boot-repair
For more information about repairing your Grub follow this link: https://help.ubuntu.com/community/Boot-Repair.
*For gpt partition table, copy the following menuentry to /etc/grub.d/40-custom/:
menuentry 'Windows Recovery Environment (on /dev/sdc1)' --class windows --class os $menuentry_id_option 'osprober-chain-5642BC722509341F' { insmod part_gpt insmod ntfs set root='hd0,gpt1' if [ x$feature_platform_search_hint = xy ]; then search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1 5642BC722509341F else search --no-floppy --fs-uuid --set=root 5642BC722509341F fi drivemap -s (hd0) ${root} chainloader +1 ntldr /bootmgr