  We propose to search for a heavy neutral lepton (HNL), that is also know as
sterile neutrino, in electron colliders running with the center-of-mass
energies at few GeV, including BESIII, Belle II, and the proposed Super Tau
Charm Factory (STCF). We consider the HNL interacting with Standard Model
neutrino and photon via a transition magnetic moment, the so-called dipole
portal.We use the monophoton signature at electron colliders to probe the
constraints on the active-sterile neutrino transition magnetic moments $d$ as
the function of the HNL's mass $m_N$.It is found that BESIII, Belle II and STCF
can probe the upper limits for $d$ down to 1.3 $\times 10^{-5}\ {\rm
GeV}^{-1}$, 8 $\times 10^{-6}\ {\rm GeV}^{-1}$, and 1.3 $\times 10^{-6}\ {\rm
GeV}^{-1}$ with $m_N$ around GeV scale, respectively, and have sensitivity to
the previously unexplored parameter space for electron- ($d_e$) and
tau-neutrino ($d_\tau$) dipole portal with $m_N$ from dozens to thounsands MeV.
On $d_\mu$ for HNL mixing with the {muon}-neutrino, Belle II and STCF can also
provide leading constraints.
