OK, finally we have an answer...
You are correctly specifying headers: {"Content-Type": "application/json"}, to set your content type. Under the hood either the package http or the lower level dart:io HttpClient is changing this to application/json; charset=utf-8. However, your server web application obviously isn't expecting the suffix.
To prove this I tried it in Java, with the two versions
conn.setRequestProperty("content-type", "application/json; charset=utf-8"); // fails
conn.setRequestProperty("content-type", "application/json"); // works
Are you able to contact the web application owner to explain their bug? I can't see where Dart is adding the suffix, but I'll look later.
EDIT
Later investigation shows that it's the http package that, while doing a lot of the grunt work for you, is adding the suffix that your server dislikes. If you can't get them to fix the server then you can by-pass http and use the dart:io HttpClient directly. You end up with a bit of boilerplate which is normally handled for you by http.
Working example below:
import 'dart:convert';
import 'dart:io';
import 'dart:async';
main() async {
String url =
'https://pae.ipportalegre.pt/testes2/wsjson/api/app/ws-authenticate';
Map map = {
'data': {'apikey': '12345678901234567890'},
};
print(await apiRequest(url, map));
}
Future<String> apiRequest(String url, Map jsonMap) async {
HttpClient httpClient = new HttpClient();
HttpClientRequest request = await httpClient.postUrl(Uri.parse(url));
request.headers.set('content-type', 'application/json');
request.add(utf8.encode(json.encode(jsonMap)));
HttpClientResponse response = await request.close();
// todo - you should check the response.statusCode
String reply = await response.transform(utf8.decoder).join();
httpClient.close();
return reply;
}
Depending on your use case, it may be more efficient to re-use the HttpClient, rather than keep creating a new one for each request. Todo - add some error handling ;-)