Populating field value with the value selected from model dialog

On my Entity form, I created a on field change event and in javascript i am opening a model dialog window, and when user make some selection from it, I return that value back to Entity form.

Entity form

function ValueChanges()
{
 var selectedvalue = window.showModalDialog("htm web resource url", arguments);

}

on htm web-resource
var receivedarugments = window.dialogArguments;

//take some decision and show values to user, and when user selects a value call selectedValue function
function selectedValue()
{
  window.returnValue = "selected value";
   window.close();
}