We demonstrate a silicon (Si) – gallium arsenide (GaAs) semiconductor based holographic metasurface antenna operating at 94 GHz. The metasurface antenna molds the radiated beam by resorting to a holographic approach involving the modulation of a guided-mode generated by a pillbox beam former. The pillbox beam former is fed by three integrated horns in Substrate Integrated Waveguide (SIW) technology located in the focal plane of the parabolic reflector of the pillbox beam former. The three horns correspond to three independent beams and allow beam steering by switching from one feed to another. The proposed metasurface antenna is planar and extremely low-profile, thus suitable for small platforms such as CubeSats/SmallSats. A prototype validates the concept demonstrating a directivity as high as 31.9 dBi and a reflection coefficient lower than -15 dB at 94 GHz.