Refresh DropDownList on parent page when a popup child updates data or it closes

I have a web form that contains a dropdownlist which shows list of cities in it. I have an add button on click of which I am opening a Popup window to open a new page that contains a text box where in i am entering new city name and submit button.

I want that after click submit button the window should be closed and on the parent page the dropdownlist gets refreshed with the new value. I am able to do it but the whole page gets postbacked which I do not want. I have added the dropdownlist in the update panel but not able to do the partial postback.

Please suggest.