The Greenworks plugin allows you to integrate your game with Steam using the Steamworks API.
If Steam is unavailable or something has gone wrong Is available will be false, so check it's true before using Steam features. There should be a message in the dev tools console if something went wrong (use the 'Show dev tools' action in the NW.js plugin to check).
Unfortunately Greenworks does not currently work in preview mode.
After you export your project for NW.js, you'll need to make the following changes before the Greenworks plugin works.
Package.nw is just a zip file with a different extension. You can open it either by renaming it to .zip, adding a file, then renaming it back to .nw (you may need to change settings to show file extensions). Alternatively you can associate the .nw file extension with a program that can open zips like WinRAR and just drag-and-drop extra files in. Note: package.nw is only created if you export with Package assets enabled. If disabled, just drop the files in to the same folder.
Download the Steamworks SDK from the Steam partner site. Make sure the version matches the one specified for the Greenworks plugin version. From the redistributable_bin folder, copy the appropriate file for the platform. For example add steam_api.dll to package.nw for the win32 build, steam_api64.dll for the win64 build, etc.
As of Greenworks for NW.js 0.19.4, you also need to copy a sdkencryptedappticket library. This can be found in public\steam\lib in the Steamworks SDK. You need to copy this to package.nw in the same way you did for steam_api.dll. For example for the win64 version, copy public\steam\lib\win64\sdkencryptedappticket64.dll to package.nw.
The steam_appid.txt file should contain nothing but your app ID on Steam. The corresponding app must also be installed on Steam. (If the app ID is not valid and installed, Greenworks won't work.) Also make sure the file has an ANSI/ASCII encoding in your text editor, otherwise it may put a UTF-8 BOM at the start of the file.
Greenworks requires Steam to be running in the background with a logged in user to work.
Hopefully everything works! If not, check the support notes above.