[AccessD] NotInList (AXP)

This is probably very simple, but I'm not understanding how to do this. 
I have a form (frmStock) with a combo box - cboFacNum.  If the number
the user wishes to use is not in the list, I need to open another form
(frmAdd) to add that number and additional information - then return to
the original form and continue on.

How do you make this work so that when you come back to frmStock to
continue on - the item you just added is in the combo box listing?

Thanks for any help.