I have not found a method of accomplishing this:
I have a layer (and related tables) in SQL server hosted on ArcGIS Server 10.6.1 being consumed in ArcGIS Online Web Map.
All this works great but I want to be able to symbolize the features based off the related table fields not the layer fields themselves. I cannot find a way to do this. (or if its even possible).
There is a GeoNet thread that describes this but the poster who originally posted a workaround was not clear and then changes his answer.... does anyone have a working method that can accomplish this?