Hi Jianxia,
I have completely the same issue like Brett. I can't see any option to add a related record in WAB (no matter if I go via Editor widget or via popup). If I click on pencil button after related table row in pop up while editing, the window with loading symbol appears and gets suck and there is no option to come back but close the pop up window and open it again. I suppose there should be at least a plus and edit buttons.
I tried to edit and add a related record into the same table (same service and webmap) through Collector and there is no problem. I can edit existing and even add a new related record.
This is my FeatureServer service URL: ost/Ji_majetek_test (FeatureServer) . Can you please try to check where is the problem?
Thank you in advance
Jane