It doesnt work that way. If you search the portal ( related child record ) for some criteria, it will return all the parent records that have child records that match your criteria.
If your goal is to only show some related records in the portal but not all of them based on some critiera, do some research on filtered portals.
Another option would be to go to the related records using Go to Related Records and then constain your found set. However this would be based on a layout of the child.