  Accreting protoplanets provide key insights into how planets assemble from
their natal protoplanetary disks. Recently, Zhou et al. (2021) used angular
differential imaging (ADI) with Hubble Space Telescope's Wide Field Camera 3
(HST/WFC3) to recover the young accreting planet PDS 70 b in F656N
($\mathrm{H}\alpha$) at a S/N of 7.9. In this paper, we demonstrate a promising
approach to efficiently imaging accreting planets by applying reference star
differential imaging (RDI) to the same dataset. We compile a reference library
from the database of WFC3 point-spread functions (PSFs) provided by Space
Telescope Science Institute and develop a set of morphology-significance
criteria for pre-selection of reference frames to improve RDI subtraction. RDI
with this PSF library results in a detection of PDS 70 b at a S/N of 5.3.
Astrometry and photometry of PDS 70 b are calibrated using a forward-modeling
method and injection-recovery tests, resulting in a separation of $186 \pm 13$
mas, a position angle of $142 \pm 5^\circ$, and an H$\alpha$ flux of $(1.7 \pm
0.3)\times10^{-15}$ erg s$^{-1}$ cm$^{-2}$. The lower detection significance
with RDI can be attributed to the $\sim$100 times lower peak-to-background
ratios of the reference PSFs compared to the ADI PSFs. Building a high-quality
reference library with WFC3 will provide unique opportunities to study
accretion variability on short timescales not limited by roll angle scheduling
constraints and efficiently search for actively accreting protoplanets in
$\mathrm{H}\alpha$ around targets inaccessible to ground-based adaptive optics
systems, such as faint transition disk hosts.
