Because recently I found myself doing the Rancher course and I wanted to try to deploy Rancher on Ubuntu Server, I decided to create a dedicated VM for that. But in our environment I did not have any machine free to create a new VM, so then I just realized that I still have the FreeNAS and TrueNAS! 

So I open the web interface of my TrueNAS, as you can see here: 

Then I open a new web page, and I download the Ubuntu Server .iso 

LINK: https://ubuntu.com/download/server

Here we will just fill up the basic info, regarding operating system, name, etc and then click next.

This part is particularly important, setup up the CPU and Memory based on the minimum requirements of that OS and your needs: 

I reccomend to create a virtual disk, in this case I assign to this disk 60 GiB

In the network interface, remember to select the VirtlO.

On this area we are gonna tick the box "Upload an installer imagine file", and than we click on choose file we search the ubuntu server .iso in our pc directory, I guess it will be downloads. After that we click upload, we wait a couple of minutes. When is ready we proceed "next".

Now click SUBMIT.

Let's wait a couple of seconds and then click on <...> VNC to open the terminal.

Proceed with the installation and when you finish everything follow these steps.