  The formation of a deeply-bound $K^-pp$ state by the $^3$He(in-flight
$K^-$,$n$) reaction is investigated theoretically in the distorted-wave impulse
approximation using the Green's function method. The expected inclusive and
semi-exclusive spectra at $p_{K^-}$ = 1.0 GeV/c and $\theta_n = 0^{\circ}$ are
calculated for the forthcoming J-PARC E15 experiment. We employ optical
potentials between the $K^-$ and ``$pp$'' core-nucleus, and demonstrate
systematically the dependence of the spectral shape on $V_0$ and $W_0$, which
are the real and imaginary parts of the strength for the optical potential,
respectively. The necessary condition to observe a distinct peak of the $K^-pp$
bound state with $I=1/2$, $J^\pi=0^-$ in the spectrum turns out to be that the
value of $V_0$ is deeper than $\sim-100$ MeV and $W_0$ shallower than
$\sim-100$ MeV, of which the strength parameters come up to recent theoretical
predictions.
