me general way, the ideal that the communication between different documents is made using message exchange.
Let’s take the following use case, the page should send a text (value of textarea) to the iFrame, the iFrame should update its content based on the received text, then should notify the parent page for it to clear the textarea.
to achieve such a result, you can make use of the window.postMessage(...) and of window.addEventListener('message', function (evt) { ... })
// html/script do iFrame
var html = `
<html>
  <head><\/head>
  <body>
    <div id="box">
      
    <\/div>
    <script type='text/javascript'>
      var box = document.getElementById('box');
      window.addEventListener('message', function (evt) {
        if (evt.origin !== 'null') // modifique o 'null' para a dominio do iframe.
          return;
        box.textContent = evt.data
        window.parent.postMessage('OK', '*') // modifique o '*' para a dominio da pagina pai.
      })
    <\/script>
  <\/body>
<\/html>
`;
// criando a pagina a ser carregada no iFrame.
var blob = new Blob([html], { type: 'text/html' })
document.querySelector('iframe').src = URL.createObjectURL(blob)
var iframe = document.querySelector('iframe')
var textarea = document.querySelector('textarea')
var button = document.querySelector('button')
// enviando uma mensagem para o iframe
button.addEventListener('click', function (evt) {
  iframe.contentWindow.postMessage(textarea.value, '*') // modifique o '*' para a dominio do iframe.
})
// recebendo uma mensagem do iframe
window.addEventListener('message', function (evt) {
  if (evt.origin !== 'null' || evt.data !== 'OK')  // modifique o 'null' para a dominio da pagina.
    return;
  textarea.value = '';
})
textarea, iframe {
  width: 300px;
  height: 120px;
}
<div>
  <textarea></textarea>
  <iframe src='#'></iframe>
</div>
<div><button>Copiar</button></div>
 
 
							
							
						 
Yes it is. It’s just below a Dropdown
– Carmona007
On the same page. Dropdowm above and Iframe below. I want to take the Dropdown value and send it to the page in the IFRAME SRC
– Carmona007
Yes it is of the Same.
– Carmona007
Yes I’m using ASP Classico.
– Carmona007
From the parent page. The Code I sent is just to get the value of the combo. Now I need to play for the iframe.
– Carmona007