  Ultraviolet observations of Ultra-hot Jupiters (UHJs), exoplanets with
temperatures over 2000\,K, provide us with an opportunity to investigate if and
how atmospheric escape shapes their upper atmosphere. Near-ultraviolet transit
spectroscopy offers a unique tool to study this process owing to the presence
of strong metal lines and a bright photospheric continuum as the light source
against which the absorbing gas is observed. WASP-189b is one of the hottest
planets discovered to date, with a day-side temperature of about 3400\,K
orbiting a bright A-type star. We present the first near-ultraviolet
observations of WASP-189b, acquired with the Colorado Ultraviolet Transit
Experiment ($CUTE$). $CUTE$ is a 6U NASA-funded ultraviolet spectroscopy
mission, dedicated to monitoring short-period transiting planets. WASP-189b was
one of the $CUTE$ early science targets and was observed during three
consecutive transits in March 2022. We present an analysis of the $CUTE$
observations and results demonstrating near-ultraviolet (2500--3300~\AA)
broadband transit depth ($1.08^{+0.08}_{-0.08}\%$) of about twice the visual
transit depth indicating that the planet has an extended, hot upper atmosphere
with a temperature of about 15000\,K and a moderate mass loss rate of about
\SI{4e8}{\kg\per\second}. We observe absorption by Mg{\sc ii} lines ($R_p/R_s$
of $0.212^{+0.038}_{-0.061}$) beyond the Roche lobe at $>$4$\sigma$
significance in the transmission spectrum at a resolution of 10~\AA, while at
lower resolution (100~\AA), we observe a quasi-continuous absorption signal
consistent with a "forest" of low-ionization metal absorption dominated by
Fe{\sc ii}. The results suggest an upper atmospheric temperature
($\sim15000$\,K), higher than that predicted by current state-of-the-art
hydrodynamic models.
