autocomplete extender not finding service method

HI

Moving from VB to C# (still learning).  I have installed Ajax control toolkit and tested it with an accordion.  Thus I know the tool kit is installed correctly.

Have added a Webservice.asmx file and added reference to the service.  Added a button and text box and with code behind can reach the webservice method which returns a value (just test junk for the moment)

page markup for test button

<div>
<asp:Button ID="Button1" runat="server"
Text="Button"
OnClick="Button1_Click"
style="height: 26px" />
<br />
<asp:Label ID="Label1" runat="server"
Text="Label">
</asp:Label>
</div>

code behind for button

protected void Button1_Click(object sender, EventArgs e)
{
WebService webService = new WebService();
List<int> lstIntegers = new List<int> { 5, 6, 7 };
Label1.Text = "Output of WebService: " + webService.Add(lstIntegers).ToString();
}

and finally web method which works.

[WebMethod]
public int Add(List<int> listInt)
{
//https://www.c-sharpcorner.com/article/how-to-create-a-web-service-project-in-net-using-visual-studio/
int result = 0;
for (int i = 0; i < listInt.Count; i++)
{
result = result + listInt[i];
}
return result;
}

I run into troubles when I add a text box and and associated autocompleteextender.  (this works ok when I program using VB)

page markup for text box and autocompleteextender

<asp:TextBox ID="ImageNameSearchTextBox" runat="server"
style="width:100%">
</asp:TextBox>
<ajaxToolkit:AutoCompleteExtender ID="ImageNameSearchTextBox_AutoCompleteExtender" runat="server"
BehaviorID="ImageNameSearchTextBox_AutoCompleteExtender"
DelimiterCharacters=""
ServicePath="~/WebService.asmx"
ServiceMethod="Get_Image_Names"
CompletionInterval="10"
completionsetcount="10"
enablecaching="true"
MinimumPrefixLength="1"
TargetControlID="ImageNameSearchTextBox">
</ajaxToolkit:AutoCompleteExtender>

and associated web method. which should return a list of letters.

public List<string> Get_Image_Names(string prefixText, int count)
{
//https://www.aspdotnet-suresh.com/2011/03/how-to-implement-auto-complete-textbox.html
List<string> my_list = new List<string>();
my_list.Add("ab");
my_list.Add("ak");
my_list.Add("kl");
my_list.Add("kk");
return my_list;
}

What am I missing?  

Thanks in advance for any help!