  A search for $B^{+} \to D_s^{+}K^{+}K^{-}$ decays is performed using $pp$
collision data corresponding to an integrated luminosity of 4.8 fb$^{-1}$,
collected at centre-of-mass energies of 7, 8 and 13$\,$TeV with the LHCb
experiment. A significant signal is observed for the first time and the
branching fraction is determined to be \begin{equation*} \mathcal{B}(B^{+} \to
D_s^{+}K^{+}K^{-} ) = (7.1 \pm 0.5 \pm 0.6 \pm 0.7) \times 10^{-6},
\end{equation*} where the first uncertainty is statistical, the second
systematic and the third due to the uncertainty on the branching fraction of
the normalisation mode $B^{+} \to D_s^{+} \overline{\kern -0.2em D}^{0}$. A
search is also performed for the pure annihilation decay $B^{+} \to
D_s^{+}\phi$. No significant signal is observed and a limit of
\begin{equation*} \mathcal{B}(B^{+} \to D_s^{+}\phi) < 4.9 \times 10^{-7}~(4.2
\times 10^{-7}) \end{equation*} is set on the branching fraction at 95$\%$
(90$\%$) confidence level.
