adding a success message to a html form with php - php

Question

I´m tring to add a success message to my form after you click submit, but i dont know how, i have tried different code from different websites, but they dont work with my code. What line do i need to add to this code please?
<form action="contact.php" method="post" id="contact">
<h1>Contact-me!</h1>
<div class="mb-3">
<label for="name" class="form-label">Name</label>
<input type="text" class="form-control" id="name" name="name" placeholder="name">
</div>
<div class="mb-3">
<label for="email" class="form-label">Email address</label>
<input type="email" class="form-control" id="email" name="email" placeholder="name#example.com">
</div>
<div class="mb-3">
<label for="message" class="form-label">Message</label>
<textarea name="message" class="form-control" id="message" rows="3"></textarea>
</div>
<div class="col-auto">
<button type="submit" class="btn btn-primary mb-3" value="SEND MESSAGE">Submit</button>
<div>
</div>
</div>
</form>
<?php
$name = $_POST['name'];
$visitor_email = $_POST['email'];
$message = $_POST['message'];
$email_from = '';
$email_subject = "New Form Submission";
$email_body = "User Name: $name.\n".
"User Email: $visitor_email.\n".
"User Message: $message.\n";
$to = "";
$headers = "From: $email_from \r\n";
$headers .= "Reply-To: $visitor_email \r\n";
mail($to,$email_subject,$email_body,$headers);
header("Location: index.html");
?>