Http Post is working in Postman, but not Flutter - flutter

Question

edit: I found the solution and answered the question.
I'm trying POST to Instagram's API in my app. The POST works fine in Postman, but in my flutter code I always get a error response: {error_type: OAuthException, code: 400, error_message: Invalid platform app} . I must be doing something wrong in my flutter POST but I can't figure out what.
Here is my Postman POST (that works every time) along with the correct body response:
And here is my Dart/Flutter code that gives me the error:
var clientID = '708XXXXXXXX';
var clientSecret = '37520XXXXXXXXXXXXX';
var grantType = 'authorization_code';
var rediUrl = 'https://httpstat.us/200';
var urlTwo = 'https://api.instagram.com/oauth/access_token';
var body = json.encode({
'client_id': clientID,
'client_secret': clientSecret,
'grant_type': grantType,
'redirect_uri': rediUrl,
'code': _accessCode
});
print('Body: $body');
var response = await http.post(urlTwo,
headers: {
'Accept': '*/*',
'Content-Type': 'application/json',
},
body: body,
);
print(json.decode(response.body));
I read about the error on a different thread and they say to post as x-www-formurlencoded.. When I switch the Content-Type to this, I get the same error.
The error I get in Flutter:
{error_type: OAuthException, code: 400, error_message: Invalid platform app}
I triple checked and I am using all the same value in Postman and Flutter. I've tried the values as Strings and numbers.
When I print body:
Body: {"client_id":"70000edited","client_secret":"37520634edited","grant_type":"authorization_code","redirect_uri":"https://httpstat.us/200","code":"AQCs-H3cIU0aF1o2KkltLuTmVMmJ-WJnZIhb9ryMqYedited"}

Answer 1

try this once
final response = await http.post(urlTwo,
body: body,
headers: {
"Accept": "application/json",
"Content-Type": "application/x-www-form-urlencoded"
},
encoding: Encoding.getByName("utf-8"));

Answer 2

I've finally got it working using a Map. Which is weird because other attempts I also used a map (I've been trying numerous stack overflow answers). Anyways, here is the working code I used:
var urlTwo = 'https://api.instagram.com/oauth/access_token';
var clientID = '7080000';
var clientSecret = '375XXXXXX';
var grantType = 'authorization_code';
var rediUrl = 'https://httpstat.us/200';
var map = new Map<String, dynamic>();
map['client_id'] = clientID;
map['client_secret'] = clientSecret;
map['grant_type'] = grantType;
map['redirect_uri'] = rediUrl;
map['code'] = _accessCode;
http.Response response = await http.post(
urlTwo,
body: map,
);
var respData = json.decode(response.body);
print(respData);