AGOL allows users access to the Related Table Fields when setting up a pop-up however, when you want to label features you don't have access to the Related Table Fields. For instance I have created a Campus Map based off of ESRI's and want to not only label a room by it's Locations Value but also with the Employee information such as Name and Extension for that office. We have a 1 to 1 relationship so there can only be one person per office area. Any help on how to address this or if it will be an enhancement in the future that would be greatly appreciated. Below is the related table fields available in the popup that I cannot access in the Manage Labels.
There is currently an enhancement logged for this!
ENH-000113953: Allow the ability to label features in a web map based on attributes in related tables in ArcGIS Online
It is still marked as New, but will hopefully be implemented in another release of AGOL. Have a great weekend!