I was not able to get it to work with using the _bygroup_ aggregation. With Viya, if you don't mind creating an aggregated table then you can accomplish this with the following steps.
1. Create the aggregated table. (Select Data menu --> icon next to current table ---> New Data from aggregation of ....
I only select id and sales for the table to make it most efficient.

2. Create the customer category named 'Aggregated Measure'

3. Create a list table and include the three data items.
