  We study the $f^+$ form factor for the $\bar B_s\to K^+\ell^-\bar\nu_\ell$
semileptonic decay in a nonrelativistic quark model. The valence quark
contribution is supplemented with a $\bar B^*$-pole term that dominates the
high $q^2$ region. To extend the quark model predictions from its region of
applicability near $q^2_{\rm max}=(M_{B_s}-M_K)^2$, we use a
multiply-subtracted Omn\`es dispersion relation. We fit the subtraction
constants to a combined input from previous light cone sum rule results in the
low $q^2$ region and the quark model results (valence plus $\bar B^*$-pole) in
the high $q^2$ region. From this analysis, we obtain $\Gamma(\bar B_s\to
K^+\ell^-\bar\nu_\ell)=(5.47^{+0.54}_{-0.46})|V_{ub}|^2\times 10^{-9}\,{\rm
MeV}$, which is about 10% and 20% higher than predictions based on Lattice QCD
and QCD light cone sum rules respectively.
