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We study the $f^+$ form factor for the semileptonic $\bar B_s\toK^+\ell^-\bar\nu_\ell$ decay in a constituent quark model. The valence quarkestimate is supplemented with the contribution from the $\bar B^*$ pole thatdominates the high $q^2$ region. We use a multiply-subtracted Omn\`esdispersion relation to extend the quark model predictions from its region ofapplicability near $q^2_{\rm max}=(M_{B_s}-M_K)^2\sim 23.75$ GeV$^2$ to all$q^2$ values accessible in the physical decay. To better constrain thedependence of $f^+$ on $q^2$, we fit the subtraction constants to a combinedinput from previous light cone sum rule [Phys. Rev. D 78 (2008) 054015] and thepresent quark model results. From this analysis, we obtain $\Gamma(\bar B_s\toK^+\ell^-\bar\nu_\ell)=(5.45^{+0.83}_{-0.80})|V_{ub}|^2\times 10^{-9}\,{\rmMeV}$, which is about 20\% higher than the prediction based only on QCD lightcone sum rule estimates. Differences are much larger for the $f^+$ form factorin the region above $q^2=15$ GeV$^2$.

$\bar B_s\to K$ semileptonic decay from an Omn\`es improved constituent quark model