I have tried to describe with 3 screenshot to defining of portlet preference field.
Workbench/Data Source click filter fields tab and add a field.
Workbench/Data Source click data source tab and edit query. In query definition window, click where/filter tab, in filters fields box click new. enter query as showen in sceernshot 2. format: [VP.your filter token name]
Finally, Dashboard/Edit Portlet click preference fields tab, you can see your field there.
IT Governance Professional & PPM Solution Architect