About : custom expression

Your request is not very clear. Anyway, try to see if autofill is what you are looking for: http://xlinesoft.com/phprunner/docs/lookup_wizard.htm

Thank you reply!
I already use Autofill But Still some problems
Because ord_no and pro_no two field Both are set lookup table and list page with search

user allow choice ord_no automatically bring out pro_no or only choice pro_no (ord_no is null)
(ord_no lookup table is order_d and autofill set Target field pro_no the source field is pro_no

and pro_no lookup table is products_m (autofill not set no use )

if ord_no use want to automatically bring out pro_no ,The next field (pro_no)Can not seem to bring out pro_no,

Save Complete Check Database Table pro_no field value is "true"
OR can write code to resolve ? AnyGood idea?

Thank you Again !

Please, could you post a screenshot of the add/edit page to clarify your request.