Ui-iframe receiving from postMessage in iframe page

I'm sure I'm missing something very simple and obvious, but I can't for the life of me see what. I've been trying to get some information from a basic page into node red by postMessage and I never get any kind of response. For example:

In the script for a page in a ui-iframe node I have this code: window.parent.postMessage("Hello from Page!", "*");

If my understanding is correct, when that is triggered, I should get a msg.payload in Node Red with "Hello from Page!". As far as I can think of, I've tried every variation on window, window.parent, parent, etc. Thank you all very much for your time! I'm still fairly new to web stuff, so if I've forgotten anything important, please let me know! Thanks again.