  Moir\'e heterostructures consisting of transition metal dichalcogenide (TMD)
hetero- and homobilayers have emerged as a promising material platform to study
correlated electronic states. Optical signatures of strong correlations in the
form of Mott-Wigner states and fractional Chern insulators have already been
observed in TMD monolayers and their twisted bilayers. In this work, we use a
moir\'e substrate containing a twisted hexagonal boron nitride (h-BN) interface
to externally generate a superlattice potential for the TMD layer: the periodic
structure of ferroelectric domains in h-BN effects a purely electrostatic
potential for charge carriers. We find direct evidence for the induced moir\'e
potential in the emergence of new excitonic resonances at integer fillings, and
our observation of an enhancement of the trion binding energy by $\simeq$ 3
meV. A theoretical model for exciton-electron interactions allows us to
directly determine the moir\'e potential modulation of 30$\pm$5 meV from the
measured trion binding energy shift. We obtain direct evidence for charge order
linked to electronic Mott-Wigner states at filling factors $\nu$ = 1/3 and
$\nu$ = 2/3 through the associated exciton Umklapp resonances.
