  We performed $^{75}$As-NMR measurements in $H\parallel ab$ to investigate the
normal-state magnetic properties of CeRh$_2$As$_2$, a recently-discovered
heavy-fermion superconductor. We compared the NMR Knight shift $K$ with the
magnetic susceptibility $\chi_{ab}$, and estimated the hyperfine coupling
constant $A_{\mathrm{hf}}$ from the slope of the $K-\chi$ plot. We observed
that the magnitude of $A_{\mathrm{hf},ab}$ at the As(1) site changes at around
20 K owing to emerging the heavy-fermion state, which was also observed in
$A_{\mathrm{hf}}$ at the As(2) site and in $H\parallel c$. The sign of
$A_{\mathrm{hf},ab}$ at the As(1) site is negative in low temperature. These
are important for the analysis of the NMR results of CeRh$_2$As$_2$ in the
superconducting state.
