This would be a very good thing to build into ArcGIS (AGOL, Pro, Desktop). It would make sense in fact to be a default behavior. @KellyGerrow
Feature Enhancements:
1. Allow automatic creation of a related record when a new origin feature is created. Allow this in AGOL.
2. Allow creation of relationships in AGOL.
3. Allow for modification of Relationship properties after creation (Desktop/AGOL) (It appears possible via JSON but allowing via GUI would be good)
4. Allow adding related records in AGOL (current workaround is using Collector or WAB Editor Widget; add full support)