Hey @reToolCurious,
You're right that util.openUrl() doesn't return a reference to the newly opened window, so calling postMessage() on it directly isn't possible using that method.
However, you might find this Retool Community post helpful—it dives into strategies for messaging between windows when embedding Retool apps:
Community Post – Embedding Retool & Messaging to Parent
Also, here's the official Retool documentation on embedding apps and passing data between parent and child contexts:
Retool Docs – Embed Apps
These resources should give you a clearer direction on implementing communication between windows in Retool. Let me know if you need help with a workaround or a custom approach!