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We request 18 orbits of HST time to observe the transit of HIP 41378 f comma a Jovian planet (9.2 Earth radii) in a long-period orbit (542 days = 1.5 years) in a multi-planet system. The host star HIP 41378 is bright (Ks = 7.7 comma V=8.9) comma Sun-like (1.2 solar masses comma 1.3 solar radii) comma and known to host at least five transiting planets. Two transits of HIP 41378 f were observed by the NASA K2 mission during non-consecutive three-month observing campaigns in 2015 and 2018 comma restricting the set of possible periods to a set of harmonics with a maximum period of roughly 1000 days and a minimum period near 50 days. An intensive radial velocity campaign (464 observations spanning four years) has just ruled out the remaining possibilities comma revealing that the HIP 41378 f has an orbital period of 542 days and an unexpectedly low mass of 12 Earth masses. HIP 41378 f is predicted to transit on 20 May 2021 and will not transit again until November 2022. We propose to observe the 2021 transit event with WFC3-G141 to accomplish three goalsdoublePoint (1) probe the atmospheric composition of a cool Jovian planet (2) investigate the possible presence of rings (3) place constraints on the presence of exomoons. HIP 41378 f occupies a poorly-probed region of parameter space and bridges the gap between the hotter planets previously studied via transmission spectroscopy and the cooler Jovian planets in our own solar system. HST transmission spectroscopy of HIP 41378 f would provide an opportunity to investigate the influence of stellar irradiation on giant planet atmospheres and inform efforts to detect Jupiter analogs with direct imaging surveys.