Check this sample
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
function ShowMessage(message) {
alert(message);
}
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick = "Button_Click" />
</form>
</body>
</html>
C#
protected void Button_Click(object sender, EventArgs e)
{
ClientScript.RegisterStartupScript(this.GetType(), "myfunction", "ShowMessage('" + TextBox1.Text + "')", true);
}