  Recently, the LHCb Collaboration reported on the evidence for a hidden charm
pentaquark state with strangeness, i.e., $P_{cs}(4459)$, in the $J/\psi\Lambda$
invariant mass distribution of the $\Xi_b^-\to J/\psi \Lambda K^-$ decay. In
this work, assuming that $P_{cs}(4459)$ is a $\bar{D}^*\Xi_c$ molecular state,
we study this decay via triangle diagrams $\Xi_b\rightarrow
\bar{D}_s^{(*)}\Xi_c\to (\bar{D}^{(*)}\bar{K})\Xi_c\to P_{cs} \bar{K}\to
(J/\psi\Lambda) \bar{K}$. Our study shows that the production yield of a spin
3/2 $\bar{D}^*\Xi_c$ state is approximately one order of magnitude larger than
that of a spin $1/2$ state due to the interference of $\bar{D}_s\Xi_c$ and
$\bar{D}_s^*\Xi_c$ intermediate states. We obtain a model independent
constraint on the product of couplings $g_{P_{cs}\bar{D}^*\Xi_c}$ and
$g_{P_{cs}J/\psi\Lambda}$. With the predictions of two particular molecular
models as inputs, we calculate the branching ratio of $\Xi_b^-\to
(P_{cs}\to)J/\psi\Lambda K^- $ and compare it with the experimental
measurement. We further predict the lineshape of this decay which could be
useful to future experimental studies.
