Combobox and Subform question

I have a combobox on my main form with a table as its rowsource.

I have a subform on my main form whose recordsource is a table.  The subform is in continuous forms format.   The table tied to the subform has 4 fields in it (Field1, Field2, Field3 and Field4).  Only fields 2 through 4 are displayed in the subform.  Field1 is not displayed.

When a user enter a new record through the subform (by scrolling to the bottom of the continous forms and entering data in the blank line, I need Field1 to be populated with the current combobox value.  If this is not done, then when the user creates a new record, Field1 will be blank because there is nowhere in the subform to manually enter data for Field1 since Field1 is not displayed.  So the question is, how do I copy the combobox value into Field1 when a user creates/adds a new record to the table using the subform.  Thanks.