LinkedIn API OAUTH returning "grant_type" error - google-apps-script

Question

I am pretty new to coding, but trying to write a simple script using LinkedIn's API that will pull an organizations follower count into google app script. Before I can even query the API, I have to authenticate using oath explained in the LinkedIn API here.
This function returns with an error response
function callLinkedAPI () {
var headers = {
"grant_type": "client_credentials",
"client_id": "78ciob33iuqepo",
"client_secret": "deCgAOhZaCrvweLs"
}
var url = `https://www.linkedin.com/oauth/v2/accessToken/`
var requestOptions = {
'method': "POST",
"headers": headers,
'contentType': 'application/x-www-form-urlencoded',
'muteHttpExceptions': true
};
var response = UrlFetchApp.fetch(url, requestOptions);
var json = response.getContentText();
var data = JSON.parse(json);
console.log(json)
}
When I try sending the headers through I get this error as a response
{"error":"invalid_request","error_description":"A required parameter \"grant_type\" is missing"}

Answer 1

grant_type, client_id, client_secret do not go in the header of the request. Instead, try to put them in the body of the POST request with the content type x-www-form-urlencoded as you already had in the headers of the code you posted.
For example:
fetch('https://www.linkedin.com/oauth/v2/accessToken/', {
method: 'POST',
headers: {
'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
},
body: new URLSearchParams({
grant_type: 'client_credentials',
client_id: '78ciob33iuqepo',
client_secret: 'deCgAOhZaCrvweLs'
})
})
.then(response => response.json())
.then(responseData => {
console.log(JSON.stringify(responseData))
})

Answer 2

Using Apps Script you should send the payload like so:
Example:
function callLinkedAPI() {
var payload = {
"grant_type": "client_credentials",
"client_id": "78ciob33iuqepo",
"client_secret": "deCgAOhZaCrvweLs"
}
var url = `https://www.linkedin.com/oauth/v2/accessToken/`
var requestOptions = {
'method': "POST",
'contentType': 'application/x-www-form-urlencoded',
'muteHttpExceptions': true,
"payload":payload
};
var response = UrlFetchApp.fetch(url, requestOptions);
var json = response.getContentText();
var data = JSON.parse(json);
console.log(json)
}