Pooled - Unmanaged Virtual desktop Collection, 'Existing Virtual Desktop' blank?

I am trying to create a Pooled - Unmanaged Virtual desktop Collection on my Windows Server 2012 R2. I have created a hyper-v virtual machine with hyper-v manager.

But when I go to the server manager to add a new collection of virtual desktop, I chose the "Pooled Virtual desktop collection" radio button and also unchecked the "Automatically create and manage virtual desktops", then I hit next and it asks me to select Existing Virtual Desktops. However the list there is blank and I don't see the VM that I created in hyper-v manager earlier.

I wonder how can I possibly fix it?

Thanks!