How do you get access to child elements inside of Wix IFrame using Wix Editor Elements ($w) ?

I tried to use the $w selector inside Corvid to access the Members Widget on my site. I can retrieve the Members Widget just fine as an wix IFrame, but I cannot drill down farther to access its children. I want to select the dropdown element inside of the Iframe. The only properties/methods I see on type IFrame in Corvid API docs are the following:

You’ll have to handle any such interactions through the HTMLComponent’s postMessage and onMessage methods, then use a querySelector or some such method to get the correct element inside your iframe.

See Corvid: Working with the HTML Element for more information.