ArcGIS Online currently does not support related records in web map pop-ups. The related records are viewable in table form in the web map, but this is not as clean or practical as the pop-up window.
The only way related records will display in the pop-up window is as a total count for each field that is associated with that specific geometry feature, which again is not practical:
Additionally, the related records are not at all supported in any of the hosted web application templates.
ArcMap displays related records very efficiently, and individually, in the identify window:
Showing related records in ArcGIS Online pop-ups is not something that should be difficult. Why can't this basic functionality be included in ArcGIS Online?