Dropdown box that update a input parameter of webservice

Hi,
I’m trying to create a dropdown box that when select one item of the list update an input parameter of webservice and refresh.
I add Data in Binding called listaBimestri for set the list that will be visualized in the dropdown box.

public List getlistaBimestri() {
if (listaBimestri == null) {
listaBimestri = new ArrayList();
listaBimestri.add(new SelectItem(“1”, “1° Bimestre”, “1”));
listaBimestri.add(new SelectItem(“2”, “2° Bimestre”, “2”));
listaBimestri.add(new SelectItem(“3”, “3° Bimestre”, “3”));
listaBimestri.add(new SelectItem(“4”, “4° Bimestre”, “4”));
listaBimestri.add(new SelectItem(“5”, “5° Bimestre”, “5”));
listaBimestri.add(new SelectItem(“6”, “6° Bimestre”, “6”));
}
return listaBimestri;
}

How can I associate the item list value to the input parameter of webservice?
For the refresh action I put the “raise on change” with the “asych hidden command”
Thanks,
Bye