In ArcGIS JS API ver. 4.x, viewing related features is currently not supported by the Feature Table widget (see the “Known Limitation” section in ArcGIS JS API official documentation here). This project demonstrates how this feature can be implemented right into the Feature Table widget using JavaScript.
A sample app has been prepared on CodePen. I have also written an article that explains the code in detail here: ArcGIS JS API ver. 4.x | View Related Features in Feature Table Widget