var xhr = new XMLHttpRequest(),
    body = JSON.stringify({"content": "", "to": ["5511960638913"]});
xhr.open("POST",'https://platform.clickatell.com/messages',true);
xhr.setRequestHeader("Content-Type", "application/json");
xhr.setRequestHeader("Authorization", "lPe91jCqTwGbErZ6OFeXjQ==");
xhr.onreadystatechange = function(){
    if (xhr.readyState == 4 && xhr.status == 200) {
        console.log('success');
    }};
xhr.send(body);