After some researched I realized there's no support for editing related tables using Web AppBuilder, this is a feature strongly necessary in our organization since we have a ton of tabular data related to polygons.
> there's no support for editing related tables using Web AppBuilder
FYI:We just enabled support for editing related tables in the Edit Widget in Web AppBuilder in ArcGIS Online. For more details, please see this blog post,
This functionality will be part of the Portal for ArcGIS 10.5 release, tentatively planned to be released in Q4 2016 or early Q1 2017.
Hope this helps,