  The computation of the form factors for the $B_s \to K \ell \nu$ decay is
presented. The b quark is treated by means of Heavy Quark Effective Theory,
currently in the static approximation. In these proceedings we discuss the
extraction of the bare matrix elements from lattice data through a combined fit
to two- and three-point correlation functions, as well as by considering
suitable ratios. The different methods agree concerning the extracted form
factors and approximately 2% accuracy is reached. The non-perturbative
renormalization and matching to QCD is described in accompanying proceedings
PoS(LATTICE2016)292.
