Solved.
DECLARE @SearchWord nvarchar(4000)
SET @SearchWord = 'tax'
EXEC('SELECT listing_id, RANK, name, address, city, zip, heading, phone
FROM listings a,
FREETEXTTABLE(listings, *, ''FormsOf(INFLECTIONAL, '+ @SearchWord +')''
)
WHERE [KEY] = a.listing_id
ORDER BY RANK DESC, name')