Hello,
as how to create your customer search field in AET, please refer to this link:
This is not exactly the same as yours. But you can refer to it.
As how to add value help to your customer field, you can create GET_V method for the search field. Maybe this is helping:
Above two hyperlinks are not the exact solution for your questions. But they can help you to understand the process.
As of how to get all contries of partners who has partner function 'Requestor', I think maybe you need to write your own codes in GET_V methods, select DB information. Please check if DBs BUT000, BUT001, BUT100, CRMM_BUT_FRG0081, etc can help.
Best Regards, Corrine