You are doing nothing wrong. In 7.2.1 we changed to window.postMessage for getting data from the PRPC content. This required us to use a callback function. I have already asked that the Mashup documentation get updated and from what I know it should be done soon. There is even an SA article I wrote thats pending a publish.
Anyway. Here is a sample solution for you trying to get pyFirstName from page OperatorID:
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