ComboBox

I am using a combo box from MetaBuilders.WebControls and it's working fine. The problem I am facing is that I can't write a client-side event like onchange or onselect so I am stuck with server side ontextchange event.

I wanted to add a feature that when user selects a person's name from the combo box list, person's address, city, state and zip should be populated in their text boxes if the record exist in the database. Also, the user can add a new person when they select 'Add a new user' from the combobox list. Like I said, it all works fine except it refreshes the page twice and it's because I am calling a server side event. Can any one tell me how I can write a client side function for this custom combo box? Thanks in advance