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JWST will launch during HST cycle 28 and begin a new era in atmospheric characterization for all types of exoplanets. Atmospheric spectroscopy of warm rocky planets comma sub-Neptune comma and Jupiter-mass planets will be obtained through a robust set of guaranteed transiting planet observations openParthe JWST ERS and GTO programsclosePar. We propose to obtain temporally-resolved UV openPar1150-3200 AngclosePar stellar spectroscopy of the 13 JWST guaranteed time targets with no UV characterization data in the HST archive. Our proposed observations will provide the UV context necessary to determine the likelihood of atmospheric formation and retention comma the identification and interpretation of atmospheric chemistry comma and the impacts of stellar activity on the exoplanet atmospheric stability.