Terrestrial Gamma-ray Flashes (TGFs) are short flashes of high energy photons, produced by thunderstorms. When interacting with the atmosphere, they produce relativistic electrons and positrons, and a part gets bounded to geomagnetic field lines and travels large distances in space. This phenomenon is called a Terrestrial Electron Beam (TEB). The Atmosphere-Space Interactions Monitor (ASIM) mounted on-board the International Space Station detected a new TEB event on March 24, 2019, originating from the tropical cyclone Johanina. Using ASIM’s low energy detector, the TEB energy spectrum is resolved down to 50 keV. We provide a method to constrain the TGF source spectrum based on the detected TEB spectrum. Applied to this event, it shows that only fully developed RREA spectra are compatible with the observation. More specifically, assuming a TGF spectrum ∝ 1/E exp(-E/ε), the compatible models have ε ≥ 6.5 MeV (E is the photon energy and ε is the cut-off energy). We could not exclude models with ε of 8 and 10 MeV.