.js

const modalUrl = '/path/to/modal/contents.html';

// This function is executed in the Channel's original iframe
function applyCallback(label) {

  // This is the magic line here. -----\
  //                                   |
  //                                   V
  const modalDom = ShopifyApp.Modal.window().window.document
  
  const form = modalDom.getElementById('VariantForm');
  SellOnAmazon.someAwesomeFunction(form);
  ShopifyApp.Modal.close();
}

//Called from within the Embedded App to open the modal.
ShopifyApp.Modal.open({
  src: modalUrl,
  title: 'My Sweet EmbeddedApp Modal',
  buttons: {
    primary: {
      label: 'Apply changes',
      callback: applyCallback
    },
    secondary: {
      label: 'Cancel',
      callback: function(label) {
        ShopifyApp.Modal.close();
      }
    }
  }
});