Window.open and pass parameters by post method

Instead of writing a form into the new window (which is tricky to get correct, with encoding of values in the HTML code), just open an empty window and post a form to it.

Example:

<form id="TheForm" method="post" action="test.asp" target="TheWindow">
<input type="hidden" name="something" value="something" />
<input type="hidden" name="more" value="something" />
<input type="hidden" name="other" value="something" />
</form>

<script type="text/javascript">
window.open('', 'TheWindow');
document.getElementById('TheForm').submit();
</script>

Edit:

To set the values in the form dynamically, you can do like this:

function openWindowWithPost(something, additional, misc) {
  var f = document.getElementById('TheForm');
  f.something.value = something;
  f.more.value = additional;
  f.other.value = misc;
  window.open('', 'TheWindow');
  f.submit();
}

To post the form you call the function with the values, like openWindowWithPost('a','b','c');.

Note: I varied the parameter names in relation to the form names to show that they don't have to be the same. Usually you would keep them similar to each other to make it simpler to track the values.

I completely agree with mercenary's answer posted above and created this function for me which works for me. It's not an answer, it's a comment on above post by mercenary

function openWindowWithPostRequest() {
  var winName='MyWindow';
  var winURL='search.action';
  var windowoption='resizable=yes,height=600,width=800,location=0,menubar=0,scrollbars=1';
  var params = { 'param1' : '1','param2' :'2'};         
  var form = document.createElement("form");
  form.setAttribute("method", "post");
  form.setAttribute("action", winURL);
  form.setAttribute("target",winName);  
  for (var i in params) {
    if (params.hasOwnProperty(i)) {
      var input = document.createElement('input');
      input.type = 'hidden';
      input.name = i;
      input.value = params[i];
      form.appendChild(input);
    }
  }              
  document.body.appendChild(form);                       
  window.open('', winName,windowoption);
  form.target = winName;
  form.submit();                 
  document.body.removeChild(form);           
}

Since you wanted the whole form inside the javascript, instead of writing it in tags, you can do this:

let windowName = 'w_' + Date.now() + Math.floor(Math.random() * 100000).toString();
var form = document.createElement("form");
form.setAttribute("method", "post");
form.setAttribute("action", "openData.do");

form.setAttribute("target", windowName);

var hiddenField = document.createElement("input"); 
hiddenField.setAttribute("type", "hidden");
hiddenField.setAttribute("name", "message");
hiddenField.setAttribute("value", "val");
form.appendChild(hiddenField);
document.body.appendChild(form);

window.open('', windowName);

form.submit();