To utilize the Steamworks WEB API, you must aquire your own Web API key from Valve. Read more here

When you’ve aquired your own Web API key, you can start using the SteamCore::Web Plugin with your own AppID. See the settings window below for more information.

SETTINGS

• AsyncActionsTimeout
  How long (in seconds) until we force timeout Async Actions (if they fail to respond)
• DisabledSubsystems
  Disabled Subsystems wont get created
• Debugging
  Will print HTTP request strings
• DevMode
  Disable HTTP requests, will only print HTTP request strings (without processing them/actually sending them)
• Key
  Steamworks Web API publisher authentication key.
• AppID
  Your AppID
• DevSteamID
  Useful when developing, set a temporary steam id to use
• SandboxMode
  If enabled, we will use the Sandbox interface ISteamMicroTxnSandbox, This interface is identical to the regular ISteamMicroTxn interface, but no actual transactions will occur.