  We present the first rest-frame optical spectrum of a high-redshift quasar
observed with JWST/NIRCam in Wide Field Slitless (WFSS) mode. The observed
quasar, J0100+2802, is the most luminous quasar known at $z>6$. We measure the
mass of the central supermassive black hole (SMBH) by means of the rest-frame
optical H$\beta$ emission line, and find consistent mass measurements of the
quasar's SMBH of $M_\bullet\approx10^{10}\,M_\odot$ when compared to the
estimates based on the properties of rest-frame UV emission lines CIV and MgII,
which are accessible from ground-based observatories. To this end, we also
present a newly reduced rest-frame UV spectrum of the quasar observed with
X-Shooter/VLT and FIRE/Magellan for a total of 16.8 hours. We readdress the
question whether this ultra-luminous quasar could be effected by strong
gravitational lensing making use of the diffraction limited NIRCam images in
three different wide band filters (F115W, F200W, F356W), which improves the
achieved spatial resolution compared to previous images taken with the Hubble
Space Telescope by a factor of two. We do not find any evidence for a
foreground deflecting galaxy, nor for multiple images of the quasar, and
determine the probability for magnification due to strong gravitational lensing
with image separations below the diffraction limit of $\Delta\theta\lesssim
0.05''$ to be $\lesssim 2.2\times 10^{-3}$. Our observations therefore confirm
that this quasar hosts a ten billion solar mass black hole less than $1$ Gyr
after the Big Bang, which is challenging to explain with current black hole
formation models.
