Pop up window using window.open

Question

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 dropdown with ItemId, ItemName.
Now I want that when I select Item2 and click button on popup form,the dropdown on the Main form should display the Item2.
I have done 1st step to open popup form.Now i need help to complete rest form,i.e to populate the dropdown from Mian form according to poup form.(I want to do this using java script)
PopUp.aspx Form

Item1Item2Item3