How to do pop up message

What tutorial did you follow for a login? If you are using the server behaviors functionality you may want to reconsider that code as it is out of date. I would recommend a CMS or looking into a provider for login functionality as this will more closely tie into your site as opposed to building things out in pieces from tutorials and instructions that may not be compatible with one another.

To display a pop-up message in Dreamweaver after a user logs in, you can use JavaScript. Here’s a simple example:

<script>
function loginSuccess() {
alert("Login successful!");
}

function loginFailure() {
alert("Login failed. Please try again.");
}
</script>

See JavaScript Popup Boxes. Decide which type of Popup you want.

• Alert,
• Confirm,
• Prompt.

https://www.w3schools.com/js/js_popup.asp

Hope that helps.