  We have chosen the $\bar{B}^0 \to D^{*+} \bar{D}^{*0} K^-$ reaction in order
to observe the $I=0, J^P=1^+$ ($R_1$) partner state of the $X_0(2866)$ stemming
from the $D^*\bar{K}^*$ molecular picture. The reaction proceeds via external
emission in the most favored Cabibbo decay mode and one observes the $R_1$
state as a very strong peak versus the background in the $D^{*+} K^-$ spectrum.
The branching ratio for $R_1$ production in this reaction is estimated of the
order of $4\times 10^{-3}$. The method used, applied to the $B^+ \to D^- D^+
K^+$ reaction, produces a ratio of signal to background in the $D^- K^+$
spectrum in very good agreement with the LHCb experiment that observed the
$X_0(2866)$.
