Automatically Adding/Expanding new row in ms access forms

i am trying to build invoice form in which user has to select item and when ever item is select from combo box new blank row should be added at bottom as per shows in below screen shot.

i have seen than Northwind sales database has done same thing but i couldn't know how to implement it. as per my search they have build form (below screen shot) and put it in Order Detail(Subform) but what is the logic behind automatically adding of new row at last? Please help