Sum on a related table field

[MathieuCain]

I have a feature layer in an ArcGIS Online map (could be point or a polygon) that has a one-many relationship with a table (previously established through a geodatabase upload). In the feature layer popup, I would like it to display the sum of the values in a single field in the related table (I've seen examples of Arcade expressions that sum the values of multiple fields in a single record, but I am interested in a more basic summary statistic of multiple records in a single field). I need this to be a dynamic calculation so that when records are updated in the related table, that sum will be updated as well. 

The follow-up question is how that calculated field will translate to the dashboard. Will it be treated as an actual field in the feature class layer (and have the zoom/pan map functionality if selected), or will it be treated as part of the related table (i.e., related tables in dashboard are not supported for zoom/pan in a map).   

[KenBuja]

You can use the FeatureSetByRelationshipName function to get the related records for your feature, then use the GroupBy function on that FeatureSet to get the sum of the values in a particular field. That would look something like this:

var relatedFeatures = FeatureSetByRelationshipName($feature, 'the relationship name', ['your field'], false)
var summary = GroupBy(relatedFeatures, '1', { name: 'Sum', expression: 'your field', statistic: 'Sum' });
return First(summary).Sum

[MathieuCain]

If I understand correctly, in the feature layer's pop-up, I add the following Arcade expression (the reason I ask is that I get a blank result every time): 

var relatedFeatures = FeatureSetByRelationshipName($feature, 'RelatedTableName', ['RelatedTableFieldName'], false)
var summary = GroupBy(relatedFeatures, '1', { name: 'Sum', expression: 'RelatedTableFieldName', statistic: 'Sum' });
return First(summary).Sum