The main purpose of a watermark is to provide information to the user about the textbox without cluttering up the page. You have probably seen many examples of
this in search textboxes in websites. When a watermarked textbox is empty, it displays a message to the user. Once the user types some text into the textbox, the
watermarked text disappears. When the user leaves the textbox the watermarked text appears again if the content of the textbox is empty.

You can easily change your textbox to provide watermark behavior by adding
onfocus and onblur event handlers. In the focus event, clear the textbox if its text
matches the watermark text. In the blur event, set the text back to watermark text if
the textbox empty.

JavaScript

function WatermarkFocus(txtElem, strWatermark) 
{
if (txtElem.value == strWatermark) txtElem.value = '';
}
function WatermarkBlur(txtElem, strWatermark) 
{
if (txtElem.value == '') txtElem.value = strWatermark;
}

ASPX

<asp:TextBox ID="txtWatermark" runat="server" />

C#

protected void Page_Load(object sender, EventArgs e)
{
string strWatermark = "Search Karamasoft.com";
txtWatermark.Text = strWatermark;
txtWatermark.Attributes.Add("onfocus", "WatermarkFocus(this, '" +
strWatermark + "');");
txtWatermark.Attributes.Add("onblur", "WatermarkBlur(this, '" +
strWatermark + "');");
}