  Identification and follow up observations of the host galaxies of fast radio
bursts (FRBs) not only help us understand the environments in which the FRB
progenitors reside, but also provide a unique way of probing the cosmological
parameters using the dispersion measures of FRBs and distances to their origin.
A fundamental requirement is an accurate distance measurement to the FRB host
galaxy, but for some sources viewed through the Galactic plane, optical/NIR
spectroscopic redshifts are extremely difficult to obtain due to dust
extinction. Here we report the first radio-based spectroscopic redshift
measurement for an FRB host galaxy, through detection of its neutral hydrogen
(HI) 21-cm emission using MeerKAT observations. We obtain an HI-based redshift
of z = 0.0357 for the host galaxy of FRB 20230718A, an apparently non-repeating
FRB detected in the CRAFT survey and localized at a Galactic latitude of -0.367
deg. Our observations also reveal that the FRB host galaxy is interacting with
a nearby companion, which is evident from the detection of an HI bridge
connecting the two galaxies. A subsequent optical spectroscopic observation
confirmed an FRB host galaxy redshift of 0.0359 +- 0.0004. This result
demonstrates the value of HI to obtain redshifts of FRBs at low Galactic
latitudes and redshifts. Such nearby FRBs whose dispersion measures are
dominated by the Milky Way can be used to characterise these components and
thus better calibrate the remaining cosmological contribution to dispersion for
more distant FRBs that provide a strong lever arm to examine the Macquart
relation between cosmological DM and redshift.
