Code :

<title></title>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server" EnableViewState ="true">
</asp:ScriptManager>
<div>
<b>ValidatorCallout Demonstration</b><br />
<table>
<tr>
<td>Name:</td>
<td><asp:TextBox runat="server" ID="NameTextBox"
BorderStyle="solid" BorderWidth="1px" BorderColor="#a9a9a9" /></td>
</tr>
<tr>
<td>Phone Number:</td>
<td><asp:TextBox runat="server" ID="PhoneNumberTextBox"
BorderStyle="solid" BorderWidth="1px" BorderColor="#a9a9a9" /></td>
</tr>
</table>
<br />
<asp:RequiredFieldValidator runat="server" ID="Req"
ControlToValidate="NameTextBox"
Display="None"
ErrorMessage="<b> Missing Field</b><br />A name is required." />
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender1" runat="server" TargetControlID="Req"
HighlightCssClass="validatorCalloutHighlight" >
</asp:ValidatorCalloutExtender>
<asp:RequiredFieldValidator runat="server" ID="PNReq"
ControlToValidate="PhoneNumberTextBox"
Display="None"
ErrorMessage="<b>Required Field Missing</b><br />A phone number is required.<div style='margin-top:5px;padding:5px;border:1px solid #e9e9e9;background-color:white;'><b>Other Options:</b><br /><a href='javascript:alert("No phone number available in
profile.");'>Extract from Profile</a></div>" />
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender2" runat="server" TargetControlID="PNReq"
HighlightCssClass="validatorCalloutHighlight"
Width="350px" >
</asp:ValidatorCalloutExtender>
<asp:RegularExpressionValidator runat="server" ID="PNRegEx"
ControlToValidate="PhoneNumberTextBox"
Display="None"
ValidationExpression="((\(\d{3}\) ?)|(\d{3}-))?\d{3}-\d{4}"
ErrorMessage="<b>Invalid Field</b><br />Please enter a phone number in the format:<br />(###) ###-####" />
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender3" runat="server" TargetControlID="PNRegEx"
HighlightCssClass="validatorCalloutHighlight">
</asp:ValidatorCalloutExtender>
<asp:Button ID="Button1" runat="server" Text="Submit"
onclick="Button1_Click" /><br /><br />
<asp:UpdatePanel runat="server" ID="up1">
<ContentTemplate>
<asp:Label id="lblMessage" runat="server" />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Button1" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
</div>
</form>
</body>

Step 10 : Now run the application by Pressing F5.

Step 11: When we define the name, Phone number and click on the Submit button then following message shows.

Resource

AJAX Text Box Watermark Extender