.js

popup.createHtmlPanel({
  url: YOUR_URL
  height: 75
})
.then(function (htmlPanel) {
  const onload = (reason) => {
    console.log(htmlPanel.window.document.getElementById("register-form"));
  };
  // if it is still loading please add event listener to DOMContentLoaded
  if(htmlPanel.window.readystate === "loading") {
    htmlPanel.window.document.addEventListener("DOMContentLoaded", () => onload("domcontentloaded") , true)
  }
  // It's past the DOMContentLoaded, the DOMContentLoaded won't be fired again. please just call the callback
  else{ 
    onload("already done");
  }
})
.catch(function (error) {
    console.log(error);
});