I have an external domain where I issue an API key to users when they sign up. How do I access this API key from a chrome extension?

I have tried storing it in my domain's local storage and then injecting an iframe from my content script, but it doesn't seem like I am able to access the API key from the iframe using something like:

iframeElement.contentWindow.localStorage.getItem("myAPIKey");

I have also tried postMessage, but the message never appears to get to my iframe. How is this issue normally solved?