<button type="submit">Log in</button>

var myButton = document.querySelector('button'); //I realize theres an easier jQuery for this
var token;

function getToken(){

        alert("get token");
        var settings = {
            "async": true,
            "crossDomain": true,
            "url": "...",
            "method": "POST",
            "headers": {
            "Content-Type": "application/json",
            "cache-control": "no-cache"
            },
            "processData": false,
            "data": "{  rn"Username":"username123",rn"Password":"password123"rn}"
        }
        alert("going to ajax");
        return $.ajax(settings);
        }

function handleData(data){

        console.log(data);
        token = data.access_token;
        console.log(token);
    }

getToken().done(handleData);

myButton.onclick = function(e){
            e.preventDefault();
            getToken();
    }