I can't find any tools in the Web map toolbar or in the Web appbuilder widgets.
I can add multipart polygon in ArcMap for the feature class, that's not a problem. In order to let the users add multipart polygons themselves, I published the feature class as a hosted service, built a webmap and then shared the map using a web application template. but there's no tool to allow the user to add multiple parts for a polygon feature.
So I guess AGOL does not support multipart polygon editing, please correct me if I am wrong...