FYI, with the release of ArcGIS Enterprise 10.5.1, Web AppBuilder embedded in Portal for ArcGIS now supports custom widgets. Please see this blog for more details,
Web AppBuilder for ArcGIS now supports Custom Widgets in ArcGIS Enterprise 10.5.1 | ArcGIS Blog
Hope this helps,
Great blog by Robert Stauder, Product Manager for GeoPlanner for ArcGIS on the new Suitability Modeler widget in Web AppBuilder. Check it out!
GeoPlanner’s Suitability Modeler is now part of the Web AppBuilder | ArcGIS Blog
Please review this blog to see all the latest updates to Web AppBuilder embedded in ArcGIS Online,
What’s New in Web AppBuilder for ArcGIS (June 2017) | ArcGIS Blog