I spoke to the brains trust and it appears the easiest way to do this is:
- Spin up another VM with the correct version of SQL Server that you want pre-installed
- Copy the Install folder for SQL Server from that new VM to your VM
- Drop the extra VM
- Proceeed with the install as normal.
This would get you around the need to have a product key as it's pre-pidded.
It is now described in the FAQ here:
I'll ask them to update the doco to mention the product keys, as I don't think that's immediately obvious from that page.