Run 1. ESDISO.bat (double click from file manager) and follow the instructions. That will give you an ISO file.
Simply install this in a VirtualBox VM as you would any other Windows ISO.
I found that using a Microsoft account didn't work because it got stuck at a step having to do with cloud file sharing, but creating a local account worked.
Finally I disabled automatic updates in system settings.
The version of Edge available has "Edge/12.10240" in the UA string, which is the EdgeHTML version, and "20.10240.16384.0" under "About this app" in the UI.
In https://github.com/mdn/browser-compat-data/pull/3309#issuecomment-455852474 @saschanaz pointed to a method for installing the first stable build of Windows 10, 10240. I've successfully done that and want to document it to be able to repeat it later.
On a Windows 10 machine:
First, download http://b1.download.windowsupdate.com/d/updt/2015/07/10240.16384.150709-1700.th1_clientpro_ret_x64fre_en-us_9d40e15f430ab89a1eabb165acbf40c9f07d18bb.esd (linked from https://www.intowindows.com/download-windows-10-rtm-esd-iso-from-microsoft-now/)
Then, download https://github.com/gus33000/ESD-Decrypter/releases/tag/4.0.0.0.fbl_prerelease(gustavem).160815-0800 (the latest release when I did this, a later one probably works too)
Run
1. ESDISO.bat
(double click from file manager) and follow the instructions. That will give you an ISO file.Simply install this in a VirtualBox VM as you would any other Windows ISO.
I found that using a Microsoft account didn't work because it got stuck at a step having to do with cloud file sharing, but creating a local account worked.
Finally I disabled automatic updates in system settings.
The version of Edge available has "Edge/12.10240" in the UA string, which is the EdgeHTML version, and "20.10240.16384.0" under "About this app" in the UI.