Yes Karen, I found a work around.
The Edit widget in the Web Appbuilder is able to merge multiple polygons into one, so you'll get a multipart polygon. the steps are:
1.Publish a polygon feature service in AGOL/ Portal
2.Build a Web map including the polygon layer in the feature service created in step 1; And define the feature template also.
3.Create a web app in Web appbuilder and choose the Web map built in previous step
4. Choose the Edit widget, enable the Merge function in the Edit widget by checking the box right to the Merge text.
5. Save and Launch the Web app and start adding polygons in the app.
6.After adding multiple polygons, use the Selection tool and select the polygons
7.Click the Union tool and you get the multipart polygon.
Hope it works for you.
Cheers,
Simo