When developing a Unity game that will be published on Facebook, you will need the Facebook SDK and an App ID. For Target Platform, you can select Gameroom (Windows) or WebGL:
You will also decide whether this is a development build or not. If it is, you will have the additional options of Autoconnect Profiler and Scripts Only Build.
Consult the following URL for instructions on how to register and configure a Facebook app: https://developers.facebook.com/docs/apps/register.