How to update a field of a parent window from a pop-up window?

How to update a field of a parent window from a pop-up window?

Hi,
 
I don't know how to do it the following.
 
I have a form ("main" form) with several fields, one of them called Total_Invoiced. Then I call another form through a Pop-Up window (I would say that is a "child" window) to insert all invoice details and to calculate the Total_amount, once I have done this I would like to return this value to the Total Invoices field of the "main" form. How can I do that?
 
I remember that in javascript you could do this using something like  parent.Total_Invoiced = Total_amount;   , is there any thing similar in Creator.
 
Thanks
 
Fidel Martín