$.ajax({
    url: "https://sf-site-here.na34.force.com/pageName",
    type: "POST",
    dataType: "json",
    ...
});

XMLHttpRequest cannot load https://sf-site-here.na34.force.com/pageName. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://external.site.com' is therefore not allowed access.