Add a shared folder on host side.

virtualbox → your machine (powered off) → settings → Shared Folders → Add new shared folder → select “Folder Path” on win10 → enter “Folder Name” = “win10_shared” → unchecked “Read-only” and “Auto-mount”.

Install VBoxLinuxAdditions on guest side.

Turn on machine → Devices → Insert Guest Addtions CD Image…

// Mount CD
$ sudo mkdir /media/cdrom
$ sudo mount -t iso9660 /dev/cdrom /media/cdrom

// Install prerequisites
$ sudo apt-get update
$ sudo apt-get install -y build-essential linux-headers-`uname -r`

// Run installation script
$ sudo /media/cdrom/./VBoxLinuxAdditions.run

// Reboot to take effect.
$ sudo shutdown -r now

Mount shared folder manually

$ mkdir ~/ubuntu_shared
$ sudo mount -t vboxsf win10_shared ~/ubuntu_shared
$ cd ~/ubuntu_shared
$ ls

Mount shared folder automatically

$ sudo nano /etc/fstab
// add the following line. (seperated by "tab", replace <username> with yours)
win10_shared    /home/<username>/ubuntu_shared  vboxsf  defaults    0   0

$ sudo nano /etc/modules
// add the following line.
vboxsf

// Reboot to take effect
$ sudo shutdown -r now

// Check shared folder is mount after boot.
$ cd ~/ubuntu_shared
$ ls