Have you tried using the Smart Editor widget? Smart Editor widget—Portal for ArcGIS | Documentation for ArcGIS Enterprise
This widget allows you to access and edit the related tables for feature layers, extending the functionality of the standard Edit widget.
I came across another GeoNet post that discusses the auto-populate functionality further: https://community.esri.com/thread/221152-auto-populate-id-using-related-tables-in-smart-editor
Hope this helps!