/*
PHP
*/
add_action( 'wp_ajax_bookmark', 'handle_bookmark' );
add_action( 'wp_ajax_nopriv_bookmark', 'handle_bookmark');

function handle_bookmark() {
    $user = $_REQUEST["user"];
    $postID = $_REQUEST["postID"];

    $res = update_field($selector, $value, $post_id);
    if($res):
        echo json_encode(array(
            "success" => true,
            "message" => "message message message"
        ));
    else:
        echo json_encode(array(
            "success" => false,
            "message" => "message message message"
        ));
    endif;

    exit();
}

/*
jQuery
*/

$.ajax({
    method: "POST",
    url: "url de ajax de wordpress",
    data: { userID: "John", postID: "Boston" }
})
.done(function( msg ) {
    console.log( "Data Saved: " + msg );
});