Dear Tetra,
you may connect the OSNR component before the fiber or before the receiver. Typically, it is connected before the receiver as the OSNR is a basic requirement to characterize the receiver.
you may refer to OptiSystem example library for guidance. Check the examples at the location C:UsersUSER NAMEDocumentsOptiSystem 19.0 SamplesAdvanced modulation systemsQAM systems16 QAM
Regards,
Ahmad