<ajaxToolkit:AutoCompleteExtender runat="server" 
            BehaviorID="AutoCompleteEx" ID="autoComplete1"
            TargetControlID="myTextBox" ServicePath="~/Services/AutoComplete.asmx"               
            ServiceMethod="GetCompletionListKeyValuePair"
            ......
            OnClientItemSelected ="IAmSelected"
        >
</ajaxToolkit:AutoCompleteExtender>

function IAmSelected( source, eventArgs ) {
   alert( " Key : "+ eventArgs.get_text() +"  Value :  "+eventArgs.get_value()); 
}

AjaxControlToolkit.AutoCompleteExtender.CreateAutoCompleteItem(<key>,<value>);

[WebMethod]

        public string[] GetCompletionList(string prefixText, int count)
        {

            if (count == 0)
            {

                count = 10;

            }

            if (prefixText.Equals("xyz"))
            {

                return new string[0];

            }

            Random random = new Random();

            List<string> items = new List<string>(count);

            for (int i = 0; i < count; i++)
            {

                char c1 = (char)random.Next(65, 90);

                char c2 = (char)random.Next(97, 122);

                char c3 = (char)random.Next(97, 122);

                items.Add(AjaxControlToolkit.AutoCompleteExtender.CreateAutoCompleteItem(prefixText + c1 + c2 + c3, i.ToString()));

            }

            return items.ToArray();

        }

The Display of the ACE does not change , it still remains the same :

Once you select an item from the DropDown of the ACE , you get an alert which shows you the value of the selectedItem.

In Your applications , instead of the Alert , process some business logic as per your requirements.

Hope this is an useful Addition.