A build of Ubuntu 20.04 LTS beta for the Hyper-V Quick Create gallery is now available.
To test, I recommend installing the 19.10 image from the Hyper-V Quick Create gallery and then substituting in the 20.04 hard drive file, the VHDX.
Get Focal Fossa VHDX
First, download and extract the zip file of the Focal Fossa daily build to C:\Users\Public\Documents\Hyper-V\Virtual hard disks.
$ aria2c -x 10 https://partner-images.canonical.com/hyper-v/desktop/focal/daily/20200407/ubuntu-desktop-focal-20200407-vhdx.zip
$ unzip ubuntu-desktop-focal-20200407-vhdx.zip -d /mnt/c/Users/Public/Documents/Hyper-V/Virtual\ hard\ disks/
Create 19.10 VM
Open Hyper-V Manager on Windows 10.
Click 'Quick Create...' in the top right corner to open the Quick Create gallery.
Select the Ubuntu 19.10 image and click 'Create Virtual Machine. Hyper-V will download and install the Ubuntu 19.10 image.
The process will finalize and you will see an option to 'Connect' or 'Edit settings'.
Substitute Focal Fossa VHDX
Click 'Edit Settings' and select 'Hard Drive' under 'SCSI Controller' on the left.
Click 'Browse', locate, and Open livecd.ubuntu-desktop-hyperv in C:\Users\Public\Documents\Hyper-V\Virtual hard disks.
Confirm the new VHDX is listed under 'Virtual hard disk' then click 'Apply' on the bottom right.
Then select 'Name' under 'Management' on the left and rename the virtual machine to 'Ubuntu 20.04 LTS Beta'.
Perform any other custom Hyper-V configuration you require, such as expanding the available RAM, click 'Apply', then 'OK'.
You will return to the 'Connect' or 'Edit settings' prompt where you can now click 'Connect'.
Then click 'Start' to start the virtual machine.
Install Ubuntu normally.
Do not select 'Log in automatically' when creating a user, this will break enhanced session.
It is okay to delete the old Ubuntu 19.10 VHDX to save space:
$ rm /mnt/c/Users/Public/Documents/Hyper-V/Virtual\ hard\ disks/Ubuntu\ 19.10.vhdx
Please follow the general guidelines in how to report WSL bugs and submit a report on this thread on the Ubuntu discourse. We are particularly interested in bugs surrounding enhanced session detection.