Display the sum of a field based on unique ID

[RobertBorchert]

I have an application that our inspectors use to inspect projects sites.

One of the readings they take is rain fall in inches.

The application has a polygon.  The polygon contains the Project ID.

The inspector selects the polygon and then selects a link to launch Survey123.  They place the pin in Survey123 and complete the inspection survey, which includes adding inches of rain if applicable.

What I would like is a way to display the sum of inches of rain for each project id.  The polygons are part of a different feature service.  This Project only contains the inspection points

$feature.PROJECT   this is the identifier for both the polygon and the inspection points. It is unique in the polygon and a number of times for the inspections. 

$feature.RAININCHES  this is the field that needs to be grouped by PROJECT and as a SUM

the end result will be viewed in the Field Maps application.

[Ed_]

Now I am not a Attribute Rules and/or Arcade expert. But maybe one way could be to look into Attribute Rules using Arcade or simply using Arcade in Field Maps or Map Viewer forms.

The following may help

ArcGIS Pro Arcade Attribute Rules

https://proceedings.esri.com/library/userconf/devsummit19/papers/DevSummitPS_64.pdf

How do I sum an attribute within an Arcade featureset created through intersects?

[RobertBorchert]

Thanks but I looked a those solutions before posting and I could not get them to work for me.