How do I get data from an iFrame and store on a Global?

Hi, I want to access an iFrame and get the data from a class therein and then put this into one of my variables? How would I do that. I tried something like this as a random stab in the dark but it didn't work..

RATE_myGlobalVar = iframe_id.getElement("fee font-body-sm font-weight-regular");

All help greatly appreciated.

Thanks