I have a related table attached to parcel data in AGOL, which I can see the related data as a result of the Query widget (such as here), though I cannot figure a way to add the related table data to the query. Only the feature service fields are listed to be add to a query expression. I suppose it is currently impossible, as I have looked everywhere that I could to find the option. The way I see it is that the related data can only be displayed as a result of the feature service query, but the related table fields cannot be added to the query expression.
