This seems to be an existing Bug: https://my.esri.com/#/support/bugs/BUG-000105597
No "+" button is visible in the Edit Widget section for related records. As OP states, only able to edit or view existing related records.
Using 10.5 ArcGIS Server Enterprise with 10.5 Portal. Not using hosted services via Data Store, just normal feature services published to ArcGIS Server.
If we can add a related record in Collector, it should not be that difficult in the Web App.
This case was Escalated and hope we get Esri Inc to come up with a hot fix of some sort.