Custom $message for user email

I need to differentiate the notification email sent to only users with a custom html template (header and footer). In my custom plugin I add this code:

add_filter ("wp_mail_content_type", "contact_mail_content_type");
    function contact_mail_content_type() { return "text/html"; }

    function custom_ninja_forms_email_user(){
    global $ninja_forms_processing;
    if( 1 == $ninja_forms_processing->get_form_ID() ) {

    ob_start();
	include("email_header.php");
	?>
	<div>MY_MESSAGE</div>
	<?php
	include("email_footer.php");
	$message = ob_get_contents();
	ob_end_clean();

	$ninja_forms_processing->update_form_setting( 'email_message', $message );
   }
}

add_action( 'ninja_forms_email_user', 'custom_ninja_forms_email_user');

Is there a way to make it work ?
Is there a specific hook for email_user ?

Thanks

https://ww.wp.xz.cn/plugins/ninja-forms/