DartPad - https://stackoverflow.com/questions/73557840/flutter-post-json-request-body-is-empty-null-on-server-side

post.dart

import 'package:http/http.dart' as http;

Future<bool> sendPost() async {
Map<String,String> headers = {
  "Accept": "application/json",
  "content-type": "application/json",
};

var response = await http.post(
    Uri.parse('https://httpbin.org/post'),
    headers:  headers,
    body: "{\"email\":\"[email protected]\",\"username\":\"\",\"mobile_phone_number\":\"\",\"password\":\"david\"}");
print(response.statusCode);
print(response.body.toString());
if(response.statusCode == 200){
  return true;
}
return false;
}

void main(){
    sendPost();
}