Buy a Windows OEM license to gain access to Windows ARM builds in order to play Elite Dangerous

It isn’t clear what MS licensing intentions for ARM are but to answer your question, you don’t need any Windows license to join the insider program - you just need to register a free MS account.

You can then download the vhdx image from windowsinsiderpreviewARM64 (just tested using freshly created MS account on my iPad) and the Program Agreement on that page doesn’t mention any license requirements.

As an aside however, after you register as an insider you’ll see on the flights tab (which seems aimed at x86 versions):

To install Windows 10 Insider Preview Builds, you must be running a licensed version of Windows 10 on your device.

This clearly doesn’t apply to M1 (as you can’t) but buying a random license wouldn’t help fulfil this condition as you wouldn’t be running it ‘on your device’.

Microsoft only licenses Windows 10 on ARM to PC makers to preinstall on new hardware, and the company hasn’t made copies of the operating system available for anyone to license or freely install.

“Microsoft only licenses Windows 10 on ARM to OEMs,” says a Microsoft spokesperson in a statement to The Verge. We asked Microsoft if it plans to change this policy to allow Windows 10 on ARM-based Macs, and the company says “we have nothing further to share at this time.”

The Verge

I have just successfully activated a Windows 11 ARM VM in Parallels 17 on an M1 Mac using a Windows 10 key that I bought a few months ago.

It should work now and apparently no special keys are needed as of October 2021.