QHttpMultiPart *multiPart = new QHttpMultiPart(QHttpMultiPart::FormDataType);

QHttpPart task;
task.setHeader(QNetworkRequest::ContentDispositionHeader, QVariant("form-data; name=\"task\""));
task.setBody("post");
multiPart->append(task);

QHttpPart board;
board.setHeader(QNetworkRequest::ContentDispositionHeader, QVariant("form-data; name=\"board\""));
board.setBody("abu");
multiPart->append(board);

QHttpPart thread;
thread.setHeader(QNetworkRequest::ContentDispositionHeader, QVariant("form-data; name=\"thread\""));
thread.setBody("42375");
multiPart->append(thread);

QHttpPart comment;
comment.setHeader(QNetworkRequest::ContentDispositionHeader, QVariant("form-data; name=\"comment\""));
comment.setBody("testus");
multiPart->append(comment);

QUrl url("https://2ch.hk/makaba/posting.fcgi");
QNetworkRequest request(url);

QNetworkAccessManager manager;
QNetworkReply *reply = manager.post(request, "&task=post&board=abu&thread=42375&comment=test"); //[1]
//QNetworkReply *reply = manager.post(request, multiPart); //[2]
multiPart->setParent(reply);

QEventLoop waitLoop;
connect(reply, &QNetworkReply::finished, &waitLoop, &QEventLoop::quit);
waitLoop.exec();

qDebug() << "reply:" << QString(reply->readAll());
reply->deleteLater();