function render_form() {
  var iframe = '<iframe class="iframe-opt-in" src="http://optin.entregaemails.com.br/accounts/54149/forms/1/contacts/new" style="border:none;width: 100%;height: 100%"></iframe>';
  document.getElementById('opt_1').innerHTML = iframe;
}

if(document.readyState == 'complete') {
  render_form();
} else {
  window.onload = render_form;
}