Hi,
Target group BP Selection search help uses component BP_HEAD_SEARCH window : SearchHelpWindow.
If you fields are added through AET then you need to do view configuration of search to make those fields available in search for filter.
If fields are created directly through custom logic, then you need to enhance the view to put custom logic for filtering the business partners.
Alternatively, if you want the search to be strictly done on custom table only then better to create custom component and view for BP search.
Hope this will be useful.
Regards,
Naresh