In Collector for ArcGIS (Windows 10), if users select a feature from the search results, then the users cannot add a related record to that feature. If users select that same feature directly from the map, then it allows users to add a related record.
If users search for a feature and select the result, it zooms to that feature. Users can exit the search window, then re-select that point directly from the map. This allows users to add related records.