A metasurface, with only a single layer of artificial atoms for ease of fabrication, can become a practical surface-equivalent route to transformation optical (TO) applications. The previous design paradigm for a metasurface carpet cloak is based on straightforward phase compensation, hampering more general wave manipulations. Here, we propose a theoretical approach in designing a metasurface using the concept of complementary media as an intermediate step. The metasurface, effectively storing all the original information in TO media, enables specific TO applications that normally require complementary media. A passive external metasurface cloak is numerically demonstrated here, which can hide an object on top of a reflective metasurface, mimicking a flat mirror. Furthermore, our scheme enables metasurfaces to be used to construct arbitrary standing waves on-demand, which will be useful for constructing tailor-made cavity modes, optical trapping, and illusion-type TO applications, which can project holograms in addition to scattering cancellation.