If you bought Windows 8.1 Single Language as a digital download (rare), the key is in your Microsoft account order history.
Because Windows 8.1 is no longer sold by Microsoft (end of sales: October 31, 2016), legitimate keys come from: windows 8.1 product key single language
Since Microsoft officially ended support for Windows 8.1 on January 10, 2023, finding activation keys can be difficult. If you bought Windows 8