For myself,There are two ways to show the user,what to be fill on the textbox..
1.in asp toolkit on you want the letter on the textbox asp tag add placeholder="abc@example.com" like runat="server"
Syntax:
<asp:TextBox ID="txtmailid" runat="server" placeholder="abc@example.com"></asp:TextBox>
Or
2. In ajax toolkit select the textboxwatermarkextender and given the targetcontrol id="id of ur textbox" and watermarktext="abc@example.com"
<asp:TextBox ID="txtmailid" runat="server"></asp:TextBox>
<ajaxToolkit:TextBoxWatermarkExtender ID="twFname" runat="server" TargetControlID="txtmailid" WatermarkText="abc@example.com" />
http://www.c-sharpcorner.com/uploadfile/rahul4_saxena/ajax-text-box-watermark-extender/[
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While in ajax toolkit dont forget to paste toolscript manager or script manager..