need help adding a javascript var to a form url

You can just change the .action property in JavaScript. Short example:

Example HTML:

<form id="myForm">
<input type="text" id="theInput" value="/script.php">
<button type="button" id="changeForm">Update Action</button>
</form>

Example JS:

(function(){
document.getElementById('changeForm').onclick = function(e) {
    document.getElementById('myForm').action = document.getElementById('theInput').value;
  }
})();

JSFiddle: JSfiddle

This is just assigning a click handler to the button that adjusts the .action property of the form "myForm" to whatever you put in the input text field.