Electrically tunable active metasurfaces are attracting great interest as a possible means to realize two-dimensional pixelated surface-normal modulators for wide applications, such as high-speed imaging, optical interconnects, and photonic switching. In this work, we fabricate a metallic metasurface embedded with an electro-optic (EO) polymer and experimentally demonstrate the electrical tuning of its reflectance property. Unlike the previously demonstrated surface-normal modulators based on the EO polymer, we utilize the Fabry-Perot resonance of a metal-insulator-metal mode to trap the incident light inside a thin EO polymer layer to enhance the modulation efficiency. By applying voltage between the top and bottom layers of Au, we observe a clear spectral shift in the plasmonic resonance as well as a 5-MHz dynamic modulation at 1630-nm wavelength.