Ajax control Target id's for list new form.aspx controls?

I have done ajaxtoolkit v4.0 registration by following link. http://ift.tt/1CXOKWS

But I didn't understand how to insert my new ajax controls instead of regular list controls in list NewForm.aspx page.

There is no specific tag for textbox (or anyother control) for inserting ajax control tag instead of that. And every control did bind with list sharePoint:FormField and SharePoint:FieldDescription tags.

                    <tr>
                    <td width="190px" valign="top" class="ms-formlabel">
                        <H3 class="ms-standardheader">
                            <nobr>Year<span class="ms-formvalidation"> *</span>
                            </nobr>
                        </H3>
                    </td>
                    <td width="400px" valign="top" class="ms-formbody">
                        <SharePoint:FormField runat="server" id="ff3{$Pos}" ControlMode="New" FieldName="Year" __designer:bind="{ddwrt:DataBind('i',concat('ff3',$Pos),'Value','ValueChanged','ID',ddwrt:EscapeDelims(string(@ID)),'@Year')}"/>
                        <SharePoint:FieldDescription runat="server" id="ff3description{$Pos}" FieldName="Year" ControlMode="New"/>
                    </td>                   

                </tr>

I just tried to put below tag under 2nd td above ..but not working. I didnt understand which id I have to pass as targetID?

<ajaxToolkit:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender1" runat="server" TargetControlID="ff3{$Pos}" WatermarkText="Enter year" />

suggestions please.. Thank you.