  The recent observation and measurement of the decay $Z_c(3900) \to \rho
\eta_c$ provides the data on the relative strength of the pion and $\rho$
coupling in the corresponding transitions between exotic resonances and pure
heavy quarkonium. It is argued that using these data, the heavy quark limit for
the $c$ and $b$ quarks, the heavy quark spin symmetry (HQSS) and the vector
dominance for photon emission by the light quarks, one can (approximately)
quantitatively estimate the rates of the radiative transitions from
$\Upsilon(5S)$ to the expected $G$-odd states of molecular bottomonium
$W_{bJ}$. The estimate of the cross section of the processes $e^+e^- \to \gamma
W_{bJ}$ at the maximum of the $\Upsilon(5S)$ resonance comes out in the
ballpark of 0.1\,pb, which sets a benchmark for a possible search for these
processes at BelleII.
