Prompt Dialog Box using java script.

<script type="text/javascript" language="javascript">
function promptFunction() {
var fVal = prompt("Enter your first name :: ", "first name goes here");
alert("Your first name is : " + fVal);
}
</script>

<asp:Button ID="btnpromptFunction" runat="server" Text="Prompt Dialog Box" OnClientClick="return promptFunction();" />