Accessing shared folder on Virtualbox 6.1 - Windows 10 host - Ubuntu 20.04 SERVER guest

With a help of great @FedonKadifeli I was able to solve the issue.

First and foremost - the guest OS is Ubuntu 20.04 LTS Server.

From a running guest OS window install Guest Additions. Go to : Devices - > Insert Guest Additions CD Image

Then to the guest OS command line :

sudo apt update
sudo apt upgrade

sudo mount /dev/sr0 /cdrom
mount: /cdrom: WARNING: device write-protected, mounted read-only. # This is OK

cd /cdrom 
sudo bash ./VBoxLinuxAdditions.run

The output of the last command should be something like that :

However, the last 3 lines of the output I got was saying this :

"This system is currently not set up to build kernel modules.
Please install the gcc make perl packages from your distribution.
VirtualBox Guest Additions: Running kernel modules will not be replaced until
the system is restarted"

So, first I had to install additional packages. And anything extra that may be needed:

sudo apt install gcc 
sudo apt install make
sudo apt install perl

Restart the guest :

reboot

Then from a Guest OS window go to :

Devices - Shared Folders - Shared Folders Settings - Add new Shared Folder :

Folder Path : C:\Users\as_to\Desktop\foo
Folder Name : foo

Mount point : /home/tomas/bar - This will create a directory after you close this window

Auto-mount (check)

Make Permanent (check)

Press OK to close the window.

Next, if you try to cd into bar you will get permission denied :

cd bar/
-bash: cd: bar/: Permission denied

Add user to the group "vboxsf" and logout/login again for the changes to be applied:

sudo usermod -aG vboxsf tomas
logout

And that's it. All should work now. Thank you again @FedonKadifeli, awesome guy.