  We present a first strong-lensing model for the galaxy cluster RM
J121218.5+273255.1 ($z=0.35$; hereafter RMJ1212; also known as Abell 1489).
This cluster is amongst the top 0.1\% richest clusters in the redMaPPer
catalog; it is significantly detected in X-ray and through the
Sunyaev-Zel'dovich effect in ROSAT and \emph{Planck} data, respectively; and
its optical luminosity distribution implies a very large lens, following
mass-to-light scaling relations. Based on these properties it was chosen for
the Webb Medium Deep Fields (WMDF) JWST/GTO program. In preparation for this
program, RMJ1212 was recently imaged with GMOS on Gemini North and in seven
optical and near-infrared bands with the \emph{Hubble Space Telescope}. We use
these data to map the inner mass distribution of the cluster, uncovering
various sets of multiple images. We also search for high-redshift candidates in
the data, as well as for transient sources. We find over a dozen high-redshift
($z\gtrsim6$) candidates based on both photometric redshift and the dropout
technique. No prominent ($\gtrsim5 \sigma$) transients were found in the data
between the two HST visits. Our lensing analysis reveals a relatively large
lens with an effective Einstein radius of $\theta_{E}\simeq32\pm3''$
($z_{s}=2$), in broad agreement with the scaling-relation expectations. RMJ1212
demonstrates that powerful lensing clusters can be selected in a robust and
automated way following the light-traces-mass assumption.
