Insert Into Another Table When A Field In Form Update

i have a form, which is based on query from multiple tables.. in this form i want to add a unbound textbox in the detail section.. whenever a user leaves this records, the system should insert a record in another table with values from this record including this field... how to do this?

the record is from a query where as the field is unbound, still we need to insert the values from record and this field at the same time into another table..

example: i have item master and item production table.... from joining these table i get a query which will give me item A and prod qty .. when i use this query as form datasource, the detail section has these two fields.. now i want to have an unbound field budget.. so i have the followig in detail sectin

item A production qty budget qty

when users enter values in budgetqty field and moves to next recod,, the system should insert into another table xyz values (itemA, production qty, budget Qty)

how to do this..which event will be good.

urgent reply is expected.