  We calculate the form factors $F_{V}(q^2,q'^2)$ and $F_{TV}(q^2,q'^2)$
describing the $B_s\to \gamma^*$ transition induced by the vector and tensor
weak currents in a broad range of values of $q^2$ and $q'^2$ far below the
quark thresholds in both $q^2$ and $q'^2$ channels. These form factors are
calculated via the distribution amplitudes of the $B_s$-meson. We then
interpolate the obtained results by a formula that contains pole at
$q'^2=M_\phi^2$ and extract the residue which gives the $B_s\to \phi$
transition form factors $V(q^2)$ and $T_1(q^2)$. In this way we obtain
theoretical predictions for these form factors without invoking quark-hadron
duality and QCD sum rules. Furthermore, we calculate the relationship between
$V(0)$ and $T_1(0)$ and the parameter $\lambda_{B_s}(\mu)$, the inverse moment
of the $B_s$-meson distribution amplitude. Using the available predictions for
$V(0)$ and $T_1(0)$ coming from approaches not referring to the $B_s$-meson
distribution amplitudes, we obtain the estimate $\lambda_{B_s}(\mu\simeq
m_b)=(0.62\pm 0.10)$ GeV.
