  We study the $B^- \to K^- \eta \eta_c$ decay by taking into account the
$S$-wave contributions from the pseudoscalar meson-pseudoscalar meson
interactions within the unitary coupled-channel approach, where the $D\bar{D}$
bound state is dynamically generated. In addition, the contribution from the
intermediate resonance $K_0^*(1430)^-$, with $K_0^*(1430)^- \to K^-\eta$, is
also considered. Our results show that there is a clear peak around $3720$~MeV
in the $\eta \eta_c$ invariant mass distribution, which could be associated
with the $D \bar{D}$ bound state. The future precise measurements of the $B^-
\to K^- \eta \eta_c$ process at the Belle II and LHCb experiments could be,
therefore, used to check the existence of the $D \bar{D}$ bound state, and to
deepen our understanding of the hadron-hadron interactions.
