website: update thankyou page (#7470)

Co-authored-by: Evgeny @ SimpleX Chat <259188159+evgeny-simplex@users.noreply.github.com>

website/src/subscribe-thankyou.html

@@ -57,6 +57,10 @@ templateEngineOverride: njk
         }
         .dark .thankyou p { color: #dfeaff; }
     </style>
+
+    <script>
+        if (window.parent !== window) window.parent.postMessage('mc:subscribed', '*');
+    </script>
 </head>
 
 <body class="bg-[#F3FAFF] dark:bg-[#000832]">