I know that you can show related tables through a link in the popup in a Portal web map or through the Web App Builder, but are do out-of-the box map application templates have this capability?
In my limited experience with Portal 10.5 they don't appear to. I have a web map containing a layer with related tables that works as expected in the web map or in an app built using the Portal WebAppBuilder. I've then built a map series story which uses the same web map and the related tables option is not showing. If I embed the WebAppBuilder app within the story map the related records do show.
Just as a quick addition to my previous reply. The related records option only seems to be available on about half of the different layouts in WebAppBuilder. Will be interesting to see if this functionality is a bit more complete in Portal 10.5.1.
I believe the Crowdsource Polling template in Portal for ArcGIS 10.5 supports showing related data.
Please see the blog for more details, https://community.esri.com/people/MKellyesri-ireland-ie-esridist/blog/2016/11/18/related-data-in-arc...
Hope this helps,