Create the form

<form id="my_form_name" name="my_form_name" method="post" action="http://www.mydomain.com/destination_page.php">
   <input type="hidden" name="form_parameter1" id="form_parameter1" value="" />
   <!-- etc... -->
</form>

Create the variable

<script type="text/javascript">
var parameter1 = 'TEST';
</script>

To load the form field with the varaible

document.getElementById('form_parameter1').value=parameter1;
//If you want to auto submit the form
document.forms["my_form_name"].submit();