  We use two independent, galaxy formation simulations, FLARES, a cosmological
hydrodynamical simulation, and SHARK, a semi-analytic model, to explore how
well the James Webb Space Telescope (JWST) will be able to uncover the
existence and parameters of the star-forming main sequence (SFS) at $z=5\to10$,
i.e. shape, scatter, normalisation. Using two independent simulations allows us
to isolate predictions (e.g., stellar mass, star formation rate, SFR,
luminosity functions) that are robust to or highly dependent on the
implementation of the physics of galaxy formation. Both simulations predict
that JWST can observe $\ge 70-90\%$ (for SHARK and FLARES respectively) of
galaxies up to $z\sim10$ (down to stellar masses of $\approx 10^{8.3}\,\rm
M_{\odot}$ and SFRs of $\approx 10^{0.5}\,\rm M_{\odot}\, yr^{-1}$) in modest
integration times and given current proposed survey areas (e.g. the Web COSMOS
$0.6\,\rm deg^2$) to accurately constrain the parameters of the SFS. Although
both simulations predict qualitatively similar distributions of stellar mass
and SFR, there are important quantitative differences, such as the abundance of
massive, star-forming galaxies, with FLARES predicting a higher abundance than
SHARK; the early onset of quenching as a result of black hole growth in FLARES
(at $z\approx 8$), not seen in SHARK until much lower redshifts; and the
implementation of synthetic photometry, with FLARES predicting more
JWST-detected galaxies ($\sim 90\%$) than SHARK ($\sim 70\%$) at $z=10$. JWST
observations will distinguish between these models, leading to a significant
improvement upon our understanding of the formation of the very first galaxies.
