Yes, Related Table is also a Feature Layer. Hence editing, adding or updating records in the Related Table is no different from editing,adding or updating a normal Feature Layer. Please use the same approach to add records as shown in Add & Delete features minus the geometry ( as I suspect your Related Table may not have any geometry associated).
Few steps here
// You will be using the ServiceFeatureTable.createFeature( <pass the attributes>) to create the Feature where ServiceFeatureTable will be your related Table
// Then you relate new feature to origin feature using feature.relateFeature()
// Finally, add new feature to related table
// Finally, perform applyEdits()