Apart from accessing an element by adding its control to the Controls Repository, there is also the possibility to access an element within a web page's Iframe via Javascript.

The procedure's steps are described below.

  1. The relevant <iframe> needs to be stored in a variable. Two sample commands:
    • var iframe = document.getElementById('TheID');
    • var iframe = document.querySelector('iframe[id="TheID"]');

  2. An element within the iframe is retrieved via the use of contentWindow.
    For instance, as
                var element = iframe.contentWindow.document.querySelector('form input[type="checkbox"]');

A relevant example can be found here.

Source: How To Get an Element in an iframe