You're on the right track, but you should review the various Arcade functions to become familiar with what each does, and what it requires.
In the case of FeatureSetByRelationshipName, the function requires the name of the relationship class, which you would find in the layer properties. When successfully executed, it still only returns a FeatureSet, which may have any number of features in it, so you need some way of returning a specific feature from the set before you'll be able to access any of its attributes.
- Josh Carlson
Kendall County GIS