Auto-populating field using spatial location of another feature and its attributes using ArcGIS Pro

[community5]

I'd like to auto-populate a field based on its spatial location of another feature. For example, I have a US County layer and a polygon feature class called "Locations" (see attached). 

When I add new polygons A & B to my "Locations" layer, how can I auto-populate the County Name from those unique spatial locations?

I've created a join between the 2 hosted feature classes, but just not sure how to make the auto-populate work?

[DavidSolari]

Attribute Rules sound like the clear answer here, you can get started at this link. If your datasets need to remain accessible in ArcMap then this is off the table, 2 things you can try are:

1. Using the new Field Maps arcade functionality to calculate the attributes client-side (link). This'll only work if your only point of entry to the data is through Field Maps.
2. Create database triggers in your RDBMS if available. This is a very fragile process as anything that changes the underlying table structure (such as enabling versioning) will break your triggers and you may have to write some complex SQL to keep the tables in line, but this'll work across the board.

[community5]

Thanks so much David for guiding me to Attribute Rules.  Looking a little closer, it appears that I would need a Standard or Advanced license to use it.  I just have a basic license.