Google Apps Script : API Error message "client_id is required" - google-apps-script

Question

I've created a variable to hold the client ID (CLIENT_ID) but I keep getting an error message saying that the client ID is required when running this function. Anyone have any idea of what I've done wrong here?
function getAuth() {
var authBasedUrl = 'https://test-api.service.hmrc.gov.uk/oauth/authorize';
var response = UrlFetchApp.fetch(authBasedUrl, {
headers: {
'Accept' : 'application/vnd.hmrc.1.0+json',
'response_type': 'code',
'client_id' : CLIENT_ID,
'scope' : 'hello',
'redirect_uri' : 'https://www.xxxxxxx.com/'
}});
var result = JSON.parse(response.getContentText());
Logger.log(JSON.stringify(result, null, 2));
}

Answer 1

Based on the docs you need to make a POST request. There is a blockqoute on the page that says:
Include the request parameters in the request body, not as request
headers.
EDIT:
function getAuth() {
var authBasedUrl = 'https://test-api.service.hmrc.gov.uk/oauth/token';
var options = {
headers: {
'Accept': 'application/vnd.hmrc.1.0+json',
"Content-Type": "application/json"
},
payload: JSON.stringify({
client_id: 'RgwU6hvdxxxxxxic6LwIt',
client_secret: '9e8c9yyyyyyyyyyc2fc2ed9126',
grant_type: 'client_credentials',
scope: 'hello'
})
}
var response = UrlFetchApp.fetch(authBasedUrl, options);
var result = JSON.parse(response.getContentText());
console.log(result);
}