How to get FedEx access token - ios

Question

I have created development account on FedEx and get all key and secret.
Now I am trying to call oauth api but didn’t get a access token but getting error as 400.
Developer link https://developer.fedex.com/api/en-us/catalog/authorization/v1/docs.html
My code is -
let headers = [
"Content-Type": "application/x-www-form-urlencoded"
]
let parameters = [
"grant_type":"client_credentials",
"client_id":"client_id",
"client_secret":"client_secret"
] as [String : String]
let postData = try? JSONSerialization.data(withJSONObject: parameters, options: [])
let request = NSMutableURLRequest(url: NSURL(string: "https://apis-sandbox.fedex.com/oauth/token")! as URL,
cachePolicy: .useProtocolCachePolicy,
timeoutInterval: 10.0)
request.httpMethod = "POST"
request.allHTTPHeaderFields = headers
request.httpBody = postData! as Data
let session = URLSession.shared
let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in
if (error != nil) {
print(error)
} else {
let httpResponse = response as? HTTPURLResponse
print(httpResponse)
}
})
dataTask.resume()
}
Error Log -
(<NSHTTPURLResponse: 0x6000009b0080> { URL: https://apis-sandbox.fedex.com/oauth/token } { Status Code: 400, Headers {
"Cache-Control" = (
"no-store"
);
"Content-Length" = (
184
);
"Content-Type" = (
"application/json"
);
Date = (
"Mon, 14 Feb 2022 10:33:53 GMT"
);
Pragma = (
"no-cache"
);
Server = (
"Layer7-API-Gateway"
);
"Set-Cookie" = (
"bm_sv=0301466DB0BFD9216A1FA61DA0EC0272~1G4H2uDnGg2TFUuh7DlMXX1JIzwt6bG0fBtlySSfQAkgYesGo8P+MKPPvGVJVn8XzadxNXeGTDpjpOqQBbG1VXH6k5geeiiBP6h+AsUvs+g8hX1utfFuyRCmuHRXGO4lZKsf6jxJU2G1plXqcPPuplkB76QJgis8P9PUvXZiMZs=; Domain=.fedex.com; Path=/; Max-Age=7131; HttpOnly"
);
"server-timing" = (
"cdn-cache; desc=MISS",
"edge; dur=651",
"origin; dur=52"
);
} })