Flutter- how to post https in this format? - dart

Question

I want to send post request in Dart.
curl looks like this.
curl -X POST "https://api-us.faceplusplus.com/facepp/v3/search" \
-F "api_key=<api_key>" \
-F "api_secret=<api_secret>" \
-F "face_token=c2fc0ad7c8da3af5a34b9c70ff764da0" \
-F "outer_id=facesetid"
I tried like this but error response says
MISSING_ARGUMENTS: api_key
final String apiKey = API_KEY;
final String apiSecret = API_SECRET;
final String faceToken = FACE_TOKEN;
final String outerId = OUTER_ID;
final data = jsonEncode({
'api_key': apiKey,
'api_secret': apiSecret,
'face_token': faceToken,
'outer_id': outerId
});
final http.Response request = await http.post(
'https://api-us.faceplusplus.com/facepp/v3/search',
body: data,
);
final String response = request.body;
final result = json.decode(response);
print(response);
print('------------------------');
print(result);
What am I missing?
How can I post this correctly?

Answer 1

From man curl
-F, --form <name=content>
(HTTP) This lets curl emulate a filled-in form in which a user has pressed the submit button. This causes curl to POST data using the Content-Type multipart/form-data
https://pub.dartlang.org/documentation/http/latest/http/MultipartRequest-class.html
var uri = Uri.parse("http://pub.dartlang.org/packages/create");
var request = new http.MultipartRequest("POST", url);
request.fields['user'] = 'nweiz#google.com';
request.files.add(new http.MultipartFile.fromFile(
'package',
new File('build/package.tar.gz'),
contentType: new MediaType('application', 'x-tar'));
request.send().then((response) {
if (response.statusCode == 200) print("Uploaded!");
});