  The detection of cyclotron resonance scattering features (CRSFs) is the only
way to directly and reliably measure the magnetic field near the surface of a
neutron star (NS). The broad energy coverage and large collection area of
\emph{Insight}-HXMT in the hard X-ray band allowed us to detect the CRSF with
the highest energy known to date, reaching about 146 keV during the 2017
outburst of the first galactic pulsing ultraluminous X-ray source (pULX) Swift
J0243.6+6124. During this outburst, the CRSF was only prominent close to the
peak luminosity $\sim 2\times10^{39}$ erg s$^{-1}$, the highest to date in any
of the Galactic pulsars. The CRSF is most significant in the spin phase region
corresponding to the main pulse of the pulse profile, and its centroid energy
evolves with phase from 120 to 146 keV. We identify this feature as the
fundamental CRSF, since no spectral feature exists at $60-70$ keV. This is the
first unambiguous detection of an electron CRSF from an ULX. We also estimate a
surface magnetic field $\sim1.6\times10^{13}$ G for Swift J0243.6+6124.
Considering that the dipole magnetic field strengths, inferred from several
independent estimates of magnetosphere radius, are at least an order of
magnitude lower than our measurement, we argue that the detection of the
highest energy CRSF reported here unambiguously proves the presence of
multipole field components close to the surface of the neutron star. Such a
scenario has previously been suggested for several pulsating ULXs, including
Swift J0243.6+6124, and our result represents the first direct confirmation of
this scenario.
