FIOS is currently not available as a TV Provider option through Apple's Single Sign-On feature. Signing up through Single Sign-On grants users immediate access to all the apps in their pay TV package on Apple TV, and all supported apps on their iPhone and iPad. This eliminates the need to log in to individual apps using your cable or satellite provider information.
Apple is continually adding new options to its list of supported TV providers. You can see a complete and updated list of available options here. It is unknown if bigger providers, such as FIOS and Comcast, will ever support Apple's Single Sign-On feature or release their own single sign-on option.