The discovery of neutrino oscillations marked a groundbreaking moment, presenting the first experimental evidence of physics beyond the Standard Model (BSM). The BSM physics exploration often involves Non-Standard Interactions (NSIs), introducing unknown neutrino couplings. Scalar NSI, a unique interaction between neutrinos and matter fermions through a scalar, manifests as a medium-dependent correction to the neutrino mass term, introducing unique phenomenology in neutrino oscillations. This study delves into the impact of scalar NSI on the measurement sensitivities of oscillation parameters in upcoming long-baseline experiments (DUNE, T2HK, T2HKK). Scalar NSI introduces medium-dependent corrections to neutrino mass terms, influencing oscillation probabilities and detector event rates. Through a synergy study among experiments (DUNE+T2HK, DUNE+T2HKK), enhanced capabilities are demonstrated in constraining scalar NSI parameters, improving sensitivity to CP-violation and mass hierarchy. The exploration of scalar NSI also emerges as a valuable avenue for constraining absolute neutrino masses in the realm of neutrino oscillation experiments.