  We present aperture masking interferometry (AMI) observations of the star HIP
65426 at $3.8\,\rm{\mu m}$ as a part of the \textit{JWST} Direct Imaging Early
Release Science (ERS) program obtained using the Near Infrared Imager and
Slitless Spectrograph (NIRISS) instrument. This mode provides access to very
small inner working angles (even separations slightly below the Michelson limit
of ${}0.5\lambda/D$ for an interferometer), which are inaccessible with the
classical inner working angles of the \textit{JWST} coronagraphs. When combined
with \textit{JWST}'s unprecedented infrared sensitivity, this mode has the
potential to probe a new portion of parameter space across a wide array of
astronomical observations. Using this mode, we are able to achieve a contrast
of $\Delta m_{F380M}{\sim }7.8$\,mag relative to the host star at a separation
of ${\sim}0.07\arcsec$ but detect no additional companions interior to the
known companion HIP\,65426\,b. Our observations thus rule out companions more
massive than $10{-}12\,\rm{M\textsubscript{Jup}}$ at separations
${\sim}10{-}20\,\rm{au}$ from HIP\,65426, a region out of reach of ground or
space-based coronagraphic imaging. These observations confirm that the AMI mode
on \textit{JWST} is sensitive to planetary mass companions orbiting at the
water frost line, even for more distant stars at $\sim$100\,pc. This result
will allow the planning and successful execution of future observations to
probe the inner regions of nearby stellar systems, opening essentially
unexplored parameter space.
