In Search Builder, you can set a rule to populate a Combobox list on the Search Panel for fields of TEXT, INTEGER, NUMBER, or REFERENCE OBJECT types.
For example, for the field Case Owner, the idea is to show only External Party users. To do this we use a custom rule to fetch the External Party records (SMPL_getPartyList).
This sample rule recovers the following fields in a JSON format:
Based on the fields in the rule, we are going to use the NAME property to show this value (Display Field), and we need the ID to filter the search (Value Field).
On runtime, the field Case Owner shows the values fetched using the rule (NAME).
Result of searching for Party_1 as the Case Owner: