Online SubSystem plugin

Use it as a normal plugin, just remove it in the PublicDependencyModuleNames of Build.cs, then when you open editor u can go to plugin and enable Online Subsytem Steam there.

Or you can edit .uproject file of your project in notepad and add this

"Plugins": [
		{
			"Name": "OnlineSubsystemSteam",
			"Enabled": true
		}
	],