Javascript XMLHTTPRequest返回错误的请求（400）-“；Require参数：grant“u type”；

对这方面的事情不熟悉，似乎找不到正确的语法。在请求后有效负载中，我需要根据我尝试访问的服务包括“grant\u type=client\u credentials”。但是，我找不到在请求负载中包含用户名和密码的JavaScript语法。以下是我到目前为止所做的（出于明显的原因，我已经删除了我的密钥和秘密）：

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提前谢谢，这几个小时我一直在忙这个

只需要从发送负载中删除null并替换为“grant\u type=client\u credentials”

var request = new XMLHttpRequest();
var path="https://ops.epo.org/3.1/auth/accesstoken";
request.onreadystatechange=state_change;

var encodedData = window.btoa("Key:secret"); //encode consumer key and secret key

request.open("POST", path, true);
request.setRequestHeader("Authorization", encodedData);
request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

request.send('grant_type=client_credentials');

function state_change() {
    if (request.readyState == "4") //Request finished and response is ready
    {
        if (request.status == "200") {
            alert("Success");
            document.write(request.responseText);;

        }
        else {
            alert("Problem retrieving data");
            console.log(this.responseXML);
        }
    }
}

var request = new XMLHttpRequest();
var path="https://ops.epo.org/3.1/auth/accesstoken";
request.onreadystatechange=state_change;

var encodedData = window.btoa("Key:secret"); //encode consumer key and secret key

request.open("POST", path, true);
request.setRequestHeader("Authorization", encodedData);
request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

request.send('grant_type=client_credentials');

function state_change() {
    if (request.readyState == "4") //Request finished and response is ready
    {
        if (request.status == "200") {
            alert("Success");
            document.write(request.responseText);;

        }
        else {
            alert("Problem retrieving data");
            console.log(this.responseXML);
        }
    }
}