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In the nearby Universe ( <100 comma Mpc) comma superluminous supernovae with peak luminosities brighter than -20 comma magnitude are extremely rare comma with only a few in total. Recently comma the Zwicky Transient Facility discovered and classified ZTF21ackxdos comma the closest and brightest SLSN-IIn in more than a decade ( z=0.018 comma 78 comma Mpc). Its current apparent brightness is 14.5 and 17.7 comma magnitudes in r and keyOpen it SwiftkeyClose UVW2 (1928virgulAA) respectively. Such high UV fluxes offer a rare and valuable opportunity for high resolution HST-UV spectroscopy. The rich suite of narrow UV emission lines from C comma N and O at various ionization states offer the best diagnostics to obtain reliable measurements of CNO abundances and electron density of the circumstellar medium (CSM) ejected from the surface of the SN progenitor star. This type of observations was carried out only once before for a luminous SN IIn comma SN comma 2010jl at z=0.01058 (Fransson et al. 2014). The requested data will determine if the nitrogen in ZTF21ackxdos is highly enriched due to the CNO-cycle comma similar to the case for SN comma 2010jl comma whose progenitor is thought to be a luminous blue variable (LBV). The high resolution COS spectra can resolve the UV lines down to la 100 comma km comma s elevkeyOpen -1keyClose and together with the density provide a measurement of the mass loss rate comma crucial for characterising this type of progenitors. ZTF21ackxdos will be visible by keyOpen it HSTkeyClose on April 7 comma 2022. We request one epoch of COS and STIS observations before the supernova becomes too faint.