'Extreme High-frequency-peaked BL Lac (EHBL) objects, a subclass of blazars characterised by a synchrotron peak frequency exceeding $10^{17}$ Hz, and, in some cases, an inverse Compton peak energy exceeding $1$ TeV, are ideal sources to study the InterGalactic Magnetic Field (IGMF) due to the hardness of their spectrum. HESS J1943+213 is a Very High Energy (VHE, larger 100 GeV) γ-ray source shining through the Galactic Plane discovered by HESS. Recently, also VERITAS published a VHE spectrum spanning from $200$ GeV up to about $2$ TeV consistent with that of HESS within the errors (photon index$=2.8$). The archetypical EHBL source is 1ES 0229+200 which has a redshift z=0.14 and a similar VHE slope (photon index$=2.9$). Since the observed flux of HESS J1943+213 at 1 TeV is more than a factor of two larger, and its redshift is bigger (z smaller $0.23$), a much larger reprocessed power is expected, which allowed us to study the magnetic field strength with great accuracy. We used the simulation code CRpropa 3 to simulate the cascade emission assuming different IGMF configurations and a detailed analysis of the $10$ years of Fermi-LAT data to extend the observed VHE spectrum down to $5$ GeV. Comparing the cascade spectrum with the combined spectra from Fermi-LAT and Cherenkov telescopes we derived a lower limit on the IGMF strength of the order of $6\cdot10^{-14}$ G which is at least a factor of $4$ larger than previously published results obtained with the source 1ES0229+200. Effects of the duty cycle are also taken into consideration.