Pointing dojo registry / widget references to iframe

How can I target an iframe in the below? I've tried incorporating dojo/request/iframe with no luck. Having zero experience in using dojo, I would appreciate any help.

require(["dijit/form/CheckBox", "dijit/registry", "dijit/form/Button", "dojo/domReady!"], function(CheckBox, registry, Button) {

  var markCheckAll = function() {
    registry.toArray().forEach(function(widget) {
      if (widget.type === 'checkbox'){
        widget.set('checked', true);
      }
    });
  };
  markCheckAll();

});

from Pointing dojo registry / widget references to iframe