We present an extended Maxwell–Hydrodynamic model of free electron dynamics on metal–dielectric interfaces that allows us to study numerically the THz emission from nonlinear metasurfaces. This model is applied on a metasurface consisting of split ring resonators, which has been previously studied and shown to produce broadband terahertz (THz) radiation. Investigations of the emitted THz radiation as function of the duration of the excitation laser reveal a tuning mechanism in terms of both spectral peak position and intensity. We also use the model to propose a new metasurface-activated waveguide platform that efficiently generates THz waveguide modes. Tunability mechanisms of the generated THz are shown. Due to its unique characteristics, we believe that this new platform might play a major role in forthcoming THz applications.