fill the dropdown on main form with the value from popup for

hello
I have a dropdown on main table ,and a select button.
Main.aspx Form
Item1Item2Item3


now when i select Item1 and click button my popup form opens,which has grid with ItemId, ItemName ,quantity.I have given Link button on item Id.
Now I want that when I click on the Link button of itemId2 in popup form,the dropdown on the Main form should display the Item with that ItemId.
I have done 1st step to open popup form.Now i need help to complete rest form,i.e to populate the dropdown.(I want to do this using java script)
PopUp.aspx Form

ItemID	ItemName	Quantity
1	Item1	12
2	Item2	23
3	Item3	32