Hi Dave,
In your sub-dialog, define a modular variable and make it PUBLIC so that it is visible outside the 4gl, and use that variable in your 4gl statement e.g.
PUBLIC DEFINE field3 CHAR(10)
...
INPUT BY NAME field3
As it is public you can reference that variable from your main-dialog.4gl
IMPORT FGL sub_dialog
...
LET m_inputRecord.field3 = sub_dialog.field3
Reuben