How do I receive data from an iframe?

Specifically, when a button is clicked in the iframe I want Construct 3 to perform a task. I know the class of the iframe button and the data in its href.

I looked at a few iframe tutorials but they seem to be sending messages from construct to the iframe and not vice versa.

Any ideas?

Check out this demo:

howtoconstructdemos.com/communicating-with-an-iframe-in-construct

Thanks, unfortunately I still can't seem to get an array of elements using the following:

let elements=iframe_main.getElementsByClassName('sv-tile__btn sv-link-reset is-primary') ;

It just returns a collection of length 0 even though I know there are about 20 elements of this class on the iFrame page.