We propose a novel approach to multiplex a single Gaussian beam from a THz source into seven beams arranged in a hexagonal configuration, matching an array of Hot Electron Bolometer (HEB) mixers. This configuration is necessary for the High-Resolution Receiver (HiRX) instrument on the proposed NASA Single Aperture Large Telescope for Universe Studies (SALTUS) space mission. The beam splitter relies on a reflector that introduces a phase shift to the incident Gaussian beam; after propagation, the desired amplitude distribution is achieved at the mixer array plane. Unlike existing THz multiplexers, our method does not use a phase grating based on the repetition of a unit cell. Instead, we employ an iterative phase reconstruction (Gerchberg- Saxton) algorithm to retrieve the required phase shift. This paper discusses the scientific motivation, current state of the art, design methodology, simulation outcomes, and experimental validation of the reflector. [...]