UI element for FaceID needed for iPhone X login? [on hold]

Many iOS apps utilize a Touch ID icon button/element on the login screen in order to invoke Touch ID in case the user has dismissed it previously, or never loaded properly within an app.

From a UX standpoint, is an element be needed for iPhone X in case FaceID hasn't been initiated properly? How should users be able to "invoke" Face ID if it's somehow dismissed or disabled while trying to launch an app?