I recently wrote a hook for this:

/**
 * Stachethemes Event Calendar 5.x mod
 * Automatically insert reminder for new attendees 2 days before event
 * @param (int) $result id of the newly created attendee
 * @return void
 * @since 5.0.7
 * 
 * @note: should be placed in the theme or child-theme (recommended) functions.php file
 */
add_action('stec_after_attendee_create_item', function ($result) {
    
    if (false === is_numeric($result) || $result <= 0) {
        return;
    }
    $event_id       = get_post_meta($result, 'event_id', true);
    $event_title    = get_post_meta($result, 'event_title', true);
    $attendee_id    = get_post_meta($result, 'attendee_id', true);
    $attendee_email = get_post_meta($result, 'attendee_email', true);
    $event_date     = get_post_meta($result, 'event_date', true); // Y-m-d\TH:i
    $attend_date    = get_post_meta($result, 'date', true); // Y-m-d\TH:i
    $remind_date    = get_post_meta($result, 'remind_date', true); // Y-m-d\TH:i
    $attend_state   = get_post_meta($result, 'state', true);
    if (!$event_id || !$event_title || !$attendee_email || !$attend_date) {
        error_log('stec_after_attendee_create_item: missing data');
        return;
    }
    if ($attend_state !== 'attending') {
        return;
    }
    $event_timezone = \Stachethemes\Stec\Helpers::get_event_timezone($event_id);
    $remind_date_object = new \DateTime($attend_date, new \DateTimeZone($event_timezone));
    $remind_date_object->sub(new \DateInterval('P2D')); // two days before the event
    $remind_date_string = $remind_date_object->format('Y-m-d\TH:i');
    $title = implode('-', array(
        (0 !== $attendee_id ? $attendee_id : $attendee_email),
        $event_id,
        $event_date,
        $remind_date
    ));
    require_once(ABSPATH . 'wp-admin/includes/post.php'); // requred for post_exists to work
    $existing_post_id = post_exists($title, '', '', 'stec_remind');
    if ($existing_post_id !== 0 && $existing_post_id !== (int) $data['id']) {
        // remind already exists
        return;
    }
    
    $remind_author = $attendee_id ? $attendee_id : 0;
    $remind_post_data = array(
        'post_type'        => 'stec_remind',
        'post_status'      => 'publish',
        'comment_status'   => 'closed',
        'post_title'       => $title,
        'post_content'     => sprintf(
            esc_html__('You have an upcoming event tomorrow: %s', 'stec'),
            $event_title
        ), // you may want to edit this message
        'author'           => $remind_author,
        'meta_input'       => array(
            'author'            => $remind_author,
            'event_id'          => $event_id,
            'event_title'       => $event_title,
            'event_date'        => $attend_date,
            'user_id'           => $attendee_id,
            'remind_date'       => $remind_date_string,
            'email'             => $attendee_email,
            'tos'               => true,
            'read_permission'   => 'stec_private',
            'edit_permission'   => 'stec_private'
        )
    );
    $result = wp_insert_post($remind_post_data);
    if (is_wp_error($result)) {
        // log error
        error_log($result->get_error_message());
    }
});

Please this hook in your theme or child-theme functions.php file.