  We bring the Kerr--Newman spacetime into the Bondi--Sachs gauge by means of
zero angular momentum, null geodesics. We compute the memory effect produced at
the black hole horizon by a transient gravitational shock wave, which from
future null infinity is seen as a Bondi-Metzner-Sachs supertranslation. This
results in a change of the supertransformation charges at infinity between the
spacetime geometries defined by the black hole before, and after, the shockwave
scattering. For an extremal Kerr--Newman black hole, we give the complementary
description of this process in the near-horizon limit, as seen by an observer
hovering over the horizon. In this limit, we compute the supertranformation
charges and compare them to those calculated at null infinity. We analyze the
effect of these transformations on the electromagnetic gauge field and explore
the self-interaction between this and the angular momentum of the black hole.
