var xhr = new XMLHttpRequest();
var body = 'grant_type=password&client_id=756665&client_secret=78c6696d3dd457e27aba3cfabd745a9f&username=test@mail.ru&password=qwerty&scope=widget';
xhr.open("POST", "https://appsmail.ru/oauth/token", true);
xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
xhr.send(body);