We report on a two-flavour lattice QCD determination of the $B_s\to D_s$
and $B_s\to D_s^*$ transitions, which in the heavy quark limit can be
parameterised by the form factors $\mathcal{G}$, and $h_{\text A_1}$, $h_{\text A_2}$ and
$h_{\text A_3}$. In the search of New Physics through tests of lepton-flavour
universality, $B_s$ decay channels are complementary to $B$ decays and
widely studied at $B$ factories and LHCb. The purpose of our study is to
explore a suitable method to extract form factors associated with $b\to c$
currents from lattice QCD. In particular, we present numerical results for
$\mathcal{G}$ and $h_{\text A_1}$.