On a case record I want to source a customer list/record field based on the only customers with specific partners. In other words, I want to display only customers who match the partners I specify. This is very dynamic, so I did the following thinking it would work.

1. Created a multi-select Partner field on the case record. This has some filtering on it that brings back specific partner records.

2. [Can't do this but want to] Create a list/record Customer field and set:
Source List: the multi-select Partner field
Source Filter By: Partner

3. On beforeload, I select all of the records in this multi-select field, so that the sourcing in #2 is activated and sources in all customers that have a partner selected in the multi-select field.