I was working on an upgrade recently, trying to move from SQL Server 2016 to SQL Server 2019. I wanted to perform an upgrade in-place on an Azure VM, but needed to get the product key. There are a few places where you can find Powershell functions to get this product key, with Ryan @ Ryadel’s post being the most well-known. That method covers versions from SQL Server 2005 through 2014 (if you follow the notes in the blog post), but breaks on 2016.
Click through for a script which works for SQL Server 2016 and later.