In this work, a design flow for leaky-wave antennas based on metasurfaces is proposed. In particular, the possibility to extract the surface impedance of partially reflecting surfaces (PRS) for which homogenization formulas are not yet available in the literature has been exploited by means of numerical tools. The knowledge of the surface impedance in a PRS-based Fabry-Perot cavity leaky-wave antenna (FPC-LWA) allows for accurately evaluating the radiating performance by means of simple analytical formulas. A fishnet-like metasurface is designed to validate the proposed approach and its implementation as a partially reflective sheet for a highly-directive FPC-LWA working at terahertz frequencies is discussed.