This would also work :
import 'package:http/http.dart' as http;
sendRequest() async {
Map data = {
'apikey': '12345678901234567890'
};
var url = 'https://pae.ipportalegre.pt/testes2/wsjson/api/app/ws-authenticate';
http.post(url, body: data)
.then((response) {
print("Response status: ${response.statusCode}");
print("Response body: ${response.body}");
});
}