Reinstate missing add_subscriber_to_legacy_form method

Author: n7studios

src/class-convertkit-api-traits.php

@@ -303,6 +303,21 @@ public function add_subscriber_to_form(int $form_id, int $subscriber_id, string
         );
     }
 
+    /**
+     * Adds a subscriber to a legacy form by subscriber ID
+     *
+     * @param integer $form_id       Legacy Form ID.
+     * @param integer $subscriber_id Subscriber ID.
+     *
+     * @since 2.0.0
+     *
+     * @return WP_Error|array
+     */
+    public function add_subscriber_to_legacy_form(int $form_id, int $subscriber_id)
+    {
+        return $this->post(sprintf('landing_pages/%s/subscribers/%s', $form_id, $subscriber_id));
+    }
+
     /**
      * List subscribers for a form
      *