jQuery POST, return output of target script

I want to submit data from an html-form using jquery (there should be no redirect when submitting). I found this snippet online and changed the var-names so it fits my needs.

<script>
    $(document).ready(function() {
        $("#submit_btn").click(function() {
            var url = $("#url").val();
            var apiKey = $("#apiKey").val();
            $("#returnmessage").empty(); // To empty previous error/success message.
            // Checking for blank fields.
            if (url == '' || apiKey == '') {
                alert("Please Fill Required Fields");
            } else {
                // Returns successful data submission message when the entered information is stored in database.
                $.post("api_test.php", {
                    url: url,
                    apiKey: apiKey,
                }, function(data) {
                    $("#returnmessage").append(data); // Append returned message to message paragraph.
                    if (data == "Your Query has been received, We will contact you soon.") {
                        $("#form")[0].reset(); // To reset form fields on success.
                    }
                });
            }
        });
    });
</script>

The target-page "api.php" returns some text (php echo). Is it possible to display the echoed text like "Please Fill Required Fields" is displayed in the alert?