I needed to send a form's data to two different locations, we couldn't edit the site, but could have them add javascript. The script scrapes out the current form action and adds a field for the page's URL. When the form is submitted, we send the data to the second submit location (set on line #22), then remove the page url field (since we don't …

form-double-sender.js

    var form = document.getElementById("form-id"); //get the form for modifications we need to make
    //get the form action and then remove the form action so we can control when the form is sent

    var action = form.action;
    form.action = "";
    console.log(`action: ${action}`);

    //create a hidden field for getting the page url, 
    //since in php the HTTP_REFERER/HTTP_ORIGIN in PHP only returns domain
    var hiddenInput = document.createElement("input");
    hiddenInput.type = "hidden";
    hiddenInput.name = "current_page_url";
    hiddenInput.value = window.location.href;
    form.appendChild(hiddenInput); //add the field fo the form

    form.addEventListener("submit", function(event) { //listen for form submit click
        event.preventDefault(); //pevent the default action so we can control the form
        
        //going to do an ajax style push of the form to a secondary location
        var formData = new FormData(form);
        var xhr = new XMLHttpRequest();
        xhr.open("POST", "https://example.com/input.php", true);
        //make sure to add a header to the form that you're submitting to cross-domain
        //header("Access-Control-Allow-Origin: *");
        xhr.onreadystatechange = function() {
            if (xhr.readyState === 4 && xhr.status === 200) {
                console.log(xhr.responseText);
            }
        };
        xhr.send(formData);
        console.log('submitted to external');
        
        //removing our hidden url field, so we don't send our extra field
        var elementsToRemove = form.querySelectorAll('input[name="current_page_url"]');
        elementsToRemove.forEach(function(element) {
            element.parentNode.removeChild(element);
        });
        console.log('extra feild removed');
        
        //now going to do the normal submit
        form.action = action;
        form.submit(); //submit the form
    });