Unable to generate OAuth 2.0 Access Token from Office365 via JavaScript - api

Question

I'm trying to pull an access token from Office365's /token identity platform endpoint via OAuth 2.0 client credentials grant flow. I have my app registered, the client ID & secret, etc...
I can make the POST request in Postman and receive the access token without issue:
However, when I try the POST request via JavaScript (by way of Google Apps Script), I receive an error message: AADSTS900144: The request body must contain the following parameter: 'grant_type'
I've already Google'd this error and found a bunch of different solutions, and have tried implementing them to no avail. I imagine this has to do with the URL encoding, but cannot figure it out.
Code:
function getO365() {
// POST Request (To get Access Token)
var tenantID = 'longstringhere'
var appID = 'longstringhere'
var appSecret = 'longstringhere'
var graphScore = 'https://graph.microsoft.com/.default'
var url = 'https://login.microsoftonline.com/' + tenantID + '/oauth2/v2.0/token'
var data = {
'client_id': appID,
'scope': graphScore,
'client_secret': appSecret,
'grant_type': 'client_credentials'
};
var postOptions = {
'method': 'POST',
'headers': {
'Accept': 'application/json',
'Content-Type': 'application/x-www-form-urlencoded'
},
'body': data,
'redirect': 'follow'
};
var authToken = UrlFetchApp.fetch(url, postOptions);
}
The only real difference between my code and the JavaScript Fetch code I pulled off of Postman is:
var urlencoded = new URLSearchParams();
urlencoded.append("client_id", "longstringhere");
urlencoded.append("scope", "https://graph.microsoft.com/.default");
urlencoded.append("client_secret", "longstringhere");
urlencoded.append("grant_type", "client_credentials");
When I try to use URLSearchParams in Google Apps Script, I keep getting this error: ReferenceError: URLSearchParams is not defined
Any ideas? Thanks in advance!

Answer 1

This was resolved by changing 'body' to 'payload' for UrlFetchApp per the documentation. Edited code to reflect the change. Credit to #TheMaster for pointing out my mistake.
'payload': data,//from 'body': data,