show pop up JavaScript message through C#

Add this following script in html code :



<script>
        function ShowMessage() {
            alert('Thank you for your submission!  To complete, please continue with PayPal transaction');
            window.location.href = 'BuyNow.aspx';
        }
   
    </script>


C# Code :-

ScriptManager.RegisterStartupScript(this, this.GetType(), "key", "ShowMessage()", true);