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In this work [V. Kocsis et al., PRB100, 155124 (2019)] we study optical magnetoelectric (ME) effect using optical directional anisotropy as measured by the THz absorption spectroscopy of spin resonances in LiCoPO₄. The antiferromagnetic domains are selected by ME poling and their population is measured by directional anisotropy [V. Kocsis et al.PRL121, 057601 (2018)]. Here we demonstrate that the directional anispotropy can also be used to investigate the form and the spectral dependence of the ME susceptibility tensor and hence to identify different spin-multipolar orders responsible for the ME effect. From the spectrum of the directional anisotropy one can determine the static ME coupling via the ME susceptibility sum rule. We conclude, for the poling magnetic field direction along the local electric polarization, the observed ME spin resonances are responsible for the static ME effect and the symmetric part of the ME tensor with zero diagonal elements dominates over the antisymmetric components.