Hi there @Rudy

Welcome to the Community!

Unfortunately, neither our support team nor the Community can troubleshoot custom forms, however, we do have some documentation on how to create and customize legacy embed forms. There is a section that details how to create a custom success message and the code in the documentation looks like what I pulled below. I would make sure to comb through the comb with a fine tooth comb to make sure there’s no additional or missing spaces to would render it unsuccessful.

<script type="text/javascript">
 KlaviyoSubscribe.attachToForms('#email_signup', {
   hide_form_on_success: true,
   success_message: "Thank you for signing up! Your special offer is on its way!"
 });
</script>

Additionally, I’d suggest checking out this similar conversation to gain more insight!

Thanks for participating in the Community!

-Taylor