How to make start date field for date in table-B depend by date which already saved in table-A ?

Hello experts,

i'm confused ..

i've two form and save in two different table.

in form-A saved in table-A. Form-A has date field (Date-A).

in form-B saved in table-B. Form-B has date field (Date-B) too.

In here, i want to set 'Start Date field ID' for Date-B dependent by Date-A value.

i put form hash variable #form.table-A.Date-A# but i recognized that  hash variable will return the value of the Date-A. But, 'Start Date field ID' ask the field id not the value..

So, how to make it right?

thankyou..