The code depends on the filter. It is normally placed where you would put a WHERE statement (or after one) as essentially that is what the filter does.
%%FILTER_USER:CONCAT(u.username,' ',u.firstname, ' ', u.lastname,' ',u.email)%%
The above filter would be used with the 'search user name details' filter, and you can free text search on username, first name, last name or email address.
Cheers
Peter