There is no single "official" standalone download, because the extension is part of a larger SDK package. Here are the legitimate ways to obtain the file:
The most reliable and safe place to download the Steamworks object is the Clickteam Community Forums. Specifically, look for the "Extensions" section or the "Released Extensions" sub-forum. steamworks.mfx download
Current Standard Version: As of recent updates, the community standard is usually the version maintained by Phi. There is no single "official" standalone download, because
Warning: Do not download .mfx files from random file-hosting sites or "DLL download" websites. These are often outdated, incompatible, or can contain malware. Always trust the Clickteam repositories or the Clickteam Extension Manager. Warning: Do not download
If you open a project (MFA file) that uses the Steamworks object and get an error saying the object is missing, it means Fusion cannot find the extension in the \Extensions\ folder.