Not clear exactly what is your expectation.
From my understanding, what you want is a search field: 1. its label should be 'country'; 2. the value help should contain all partners' countries, and those partners should have a partner function 'Requester'.
Is my understanding correct?
What is your confusion? Is it a technical problem that you want to know how to add a search field? Or is it a function problem that you want to know how to get all countries from the partners who have a partner function 'Requester' ?