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.
If you are the developer trying to export your game:
Unlike a standard .exe or .dll, steamworks.mfx is proprietary code that links directly to Valve’s API. Clickteam does not host a public list of individual .mfx files for download because:
If a website offers a raw steamworks.mfx file for download, treat it as malware. Hackers often rename viruses to common game engine extensions to trick developers.
Unlike standard extensions that come bundled with the software, the Steamworks extension often requires a manual download to ensure you have the version compatible with the latest Steam SDK.