Ajax ComboBox Validation in JavaScript

var cmb = document.getElementById('<%=cmbwno.ClientID %>');
 var string = cmb.control._originalSelectedIndex;
  if (string == 0) {
     alert("Select Worker Number");
      document.getElementById('<%=cmbwno.ClientID %>').focus();
      return false;
  }
         
but Here combobox must be auto postback  true