Follow this code you'll achieve your goal.
function onFocus() {
var txtBox = document.getElementById('<%=TextBox1.ClientID %>');
if (txtBox.value == "User Name") {
txtBox.value = "";
txtBox.style.backgroundColor = "white";
txtBox.style.color = "Black"
}
}
function onFoucsOut() {
var txtBox = document.getElementById('<%=TextBox1.ClientID %>');
if (txtBox.value == "") {
txtBox.value = "User Name";
txtBox.style.backgroundColor = "Azure";
txtBox.style.color = "Silver";
}
}
<blockquote class="FQ"><div class="FQA">Quote:</div> ></blockquote>