  We report observations of the recently discovered warm Neptune TOI-674 b
(5.25 \rearth{}, 23.6 \mearth{}) with the Hubble Space Telescope's Wide Field
Camera 3 instrument. TOI-674 b is in the Neptune desert, an observed paucity of
Neptune-size exoplanets at short orbital periods. Planets in the desert are
thought to have complex evolutionary histories due to photoevaporative mass
loss or orbital migration, making identifying the constituents of their
atmospheres critical to understanding their origins. We obtained near-infrared
transmission spectroscopy of the planet's atmosphere with the G141 grism. After
extracting, detrending, and fitting the spectral lightcurves to measure the
planet's transmission spectrum, we used the petitRADTRANS atmospheric spectral
synthesis code to perform retrievals on the planet's atmosphere to identify
which absorbers are present. These results show moderate evidence for increased
absorption at 1.4 $\mu$m due to water vapor at 2.9$\sigma$ (Bayes factor =
15.8), as well as weak evidence for the presence of clouds at 2.2$\sigma$
(Bayes factor = 4.0). TOI-674 b is a strong candidate for further study to
refine the water abundance, which is poorly constrained by our data. We also
incorporated new TESS short-cadence optical photometry, as well as Spitzer/IRAC
data, and re-fit the transit parameters for the planet. We find the planet to
have the following transit parameters: $R_p/R_* = 0.1135\pm0.0006$, $T_0 =
2458544.523792\pm0.000452$ BJD, and $P = 1.977198\pm0.00007$ d. These
measurements refine the planet radius estimate and improve the orbital
ephemerides for future transit spectroscopy observations of this highly
intriguing warm Neptune.
